diff --git a/app/controllers/application_controller.rb b/app/controllers/application_controller.rb
--- a/app/controllers/application_controller.rb
+++ b/app/controllers/application_controller.rb
@@ -152,8 +152,9 @@
   def is_request_ip_allowed?
     unless GraderConfiguration[WHITELIST_IGNORE_CONF_KEY]
       user_ip = IPAddr.new(request.remote_ip)
+      allowed = GraderConfiguration[WHITELIST_IP_CONF_KEY] || ''
 
-      GraderConfiguration[WHITELIST_IP_CONF_KEY].delete(' ').split(',').each do |ips|
+      allowed.delete(' ').split(',').each do |ips|
         allow_ips = IPAddr.new(ips)
         if allow_ips.include?(user_ip)
           return true
diff --git a/app/controllers/groups_controller.rb b/app/controllers/groups_controller.rb
--- a/app/controllers/groups_controller.rb
+++ b/app/controllers/groups_controller.rb
@@ -2,6 +2,7 @@
   before_action :set_group, only: [:show, :edit, :update, :destroy,
                                    :add_user, :remove_user,:remove_all_user,
                                    :add_problem, :remove_problem,:remove_all_problem,
+                                   :toggle,
                                   ]
   before_action :admin_authorization
 
@@ -49,6 +50,11 @@
     redirect_to groups_url, notice: 'Group was successfully destroyed.'
   end
 
+  def toggle
+    @group.enabled = @group.enabled? ? false : true
+    @group.save
+  end
+
   def remove_user
     user = User.find(params[:user_id])
     @group.users.delete(user)
@@ -99,6 +105,6 @@
 
     # Only allow a trusted parameter "white list" through.
     def group_params
-      params.require(:group).permit(:name, :description)
+      params.require(:group).permit(:name, :description, :enabled)
     end
 end
diff --git a/app/controllers/main_controller.rb b/app/controllers/main_controller.rb
--- a/app/controllers/main_controller.rb
+++ b/app/controllers/main_controller.rb
@@ -188,7 +188,7 @@
       @user = user
     end
   end
-  
+
   protected
 
   def prepare_announcements(recent=nil)
diff --git a/app/controllers/submissions_controller.rb b/app/controllers/submissions_controller.rb
--- a/app/controllers/submissions_controller.rb
+++ b/app/controllers/submissions_controller.rb
@@ -16,8 +16,8 @@
     else
       @problem = Problem.find_by_id(params[:problem_id])
       if (@problem == nil) or (not @problem.available)
-        redirect_to main_list_path
-        flash[:notice] = 'Error: submissions for that problem are not viewable.'
+        redirect_to list_main_path
+        flash[:error] = 'Authorization error: You have no right to view submissions for this problem'
         return
       end
       @submissions = Submission.find_all_by_user_problem(@user.id, @problem.id).order(id: :desc)
@@ -94,9 +94,8 @@
 
   def submission_authorization
     #admin always has privileged
-    if @current_user.admin?
-      return true
-    end
+    return true if @current_user.admin?
+    return true if @current_user.has_role?('TA') && (['show','download'].include? action_name)
 
     sub = Submission.find(params[:id])
     if @current_user.available_problems.include? sub.problem
diff --git a/app/controllers/tasks_controller.rb b/app/controllers/tasks_controller.rb
--- a/app/controllers/tasks_controller.rb
+++ b/app/controllers/tasks_controller.rb
@@ -27,8 +27,9 @@
   def download
     problem = Problem.find(params[:id])
     unless @current_user.can_view_problem? problem
-      flash[:notice] = 'You are not authorized to access this file'
-      redirect_to :action => 'index' and return
+      flash[:error] = 'You are not authorized to access this file'
+      redirect_to list_main_path
+      return
     end
 
     base_name = params[:file]
@@ -37,7 +38,8 @@
 
     if !FileTest.exists?(filename)
       flash[:notice] = 'File does not exists'
-      redirect_to :action => 'index' and return
+      redirect_to list_main_path
+      return
     end
 
     send_file_to_user(filename, base_filename)
diff --git a/app/controllers/user_admin_controller.rb b/app/controllers/user_admin_controller.rb
--- a/app/controllers/user_admin_controller.rb
+++ b/app/controllers/user_admin_controller.rb
@@ -58,7 +58,8 @@
     ok_user = []
 
     lines.split("\n").each do |line|
-      items = line.chomp.split(',')
+      #split with large limit, this will cause consecutive ',' to be result in a blank
+      items = line.chomp.split(',',1000)
       if items.length>=2
         login = items[0]
         full_name = items[1]
@@ -66,8 +67,12 @@
         user_alias = ''
 
         added_random_password = false
-        if items.length >= 3 and items[2].chomp(" ").length > 0;
-          password = items[2].chomp(" ")
+        added_password = false
+        if items.length >= 3
+          if items[2].chomp(" ").length > 0
+            password = items[2].chomp(" ")
+            added_password = true
+          end
         else
           password = random_password
           added_random_password=true;
@@ -79,16 +84,21 @@
           user_alias = login
         end
 
+
+        has_remark = false
         if items.length>=5
           remark = items[4].strip;
+          has_remark = true
         end
 
         user = User.find_by_login(login)
         if (user)
           user.full_name = full_name
-          user.password = password
-          user.remark = remark
+          user.remark = remark if has_remark
+          user.password = password if added_password || added_random_password
         else
+          #create a random password if none are given
+          password = random_password unless password
           user = User.new({:login => login,
                            :full_name => full_name,
                            :password => password,
@@ -345,36 +355,33 @@
   # admin management
 
   def admin
-    @admins = User.all.find_all {|user| user.admin? }
+    @admins = Role.where(name: 'admin').take.users
+    @tas = Role.where(name: 'ta').take.users
   end
 
-  def grant_admin
-    login = params[:login]
-    user = User.find_by_login(login)
-    if user!=nil
-      admin_role = Role.find_by_name('admin')
-      user.roles << admin_role
-    else
-      flash[:notice] = 'Unknown user'
+  def modify_role
+    user = User.find_by_login(params[:login])
+    role = Role.find_by_name(params[:role])
+    unless user && role
+      flash[:error] = 'Unknown user or role'
+      redirect_to admin_user_admin_index_path
+      return
     end
-    flash[:notice] = 'User added as admins'
-    redirect_to :action => 'admin'
-  end
-
-  def revoke_admin
-    user = User.find(params[:id])
-    if user==nil
-      flash[:notice] = 'Unknown user'
-      redirect_to :action => 'admin' and return
-    elsif user.login == 'root'
-      flash[:notice] = 'You cannot revoke admisnistrator permission from root.'
-      redirect_to :action => 'admin' and return
+    if params[:commit] == 'Grant'
+      #grant role
+      user.roles << role
+      flash[:notice] = "User '#{user.login}' has been granted the role '#{role.name}'"
+    else
+      #revoke role
+      if user.login == 'root' && role.name == 'admin'
+        flash[:error] = 'You cannot revoke admisnistrator permission from root.'
+        redirect_to admin_user_admin_index_path
+        return
+      end
+      user.roles.delete(role)
+      flash[:notice] = "The role '#{role.name}' has been revoked from User '#{user.login}'"
     end
-
-    admin_role = Role.find_by_name('admin')
-    user.roles.delete(admin_role)
-    flash[:notice] = 'User permission revoked'
-    redirect_to :action => 'admin'
+    redirect_to admin_user_admin_index_path
   end
 
   # mass mailing
diff --git a/app/helpers/application_helper.rb b/app/helpers/application_helper.rb
--- a/app/helpers/application_helper.rb
+++ b/app/helpers/application_helper.rb
@@ -113,8 +113,9 @@
 
   def toggle_button(on,toggle_url,id, option={})
     btn_size = option[:size] || 'btn-xs'
+    btn_block = option[:block] || 'btn-block'
     link_to (on ? "Yes" : "No"), toggle_url,
-      {class: "btn btn-block #{btn_size} btn-#{on ? 'success' : 'default'} ajax-toggle",
+      {class: "btn #{btn_block} #{btn_size} btn-#{on ? 'success' : 'default'} ajax-toggle",
         id: id,
         data: {remote: true, method: 'get'}}
   end
@@ -181,9 +182,6 @@
 #{header}
 <tr>
 <td class="left-col">
-#{user.full_name}<br/>
-#{t 'title_bar.current_time'} #{format_short_time(Time.zone.now)}
-#{time_left}
 <br/>
 </td>
 <td class="right-col">#{contest_name}</td>
diff --git a/app/helpers/main_helper.rb b/app/helpers/main_helper.rb
--- a/app/helpers/main_helper.rb
+++ b/app/helpers/main_helper.rb
@@ -1,17 +1,11 @@
 module MainHelper
 
-  def link_to_description_if_any(name, problem, options={})
+  def link_to_description_if_any(name, problem)
     if !problem.url.blank?
-      return link_to name, problem.url, options
+      return link_to name, problem.url
     elsif !problem.description_filename.blank?
-      #build a link to a problem (via task controller)
       basename, ext = problem.description_filename.split('.')
-      options[:controller] = 'tasks'
-      options[:action] = 'download'
-      options[:id] = problem.id
-      options[:file] = basename
-      options[:ext] = ext
-      return link_to name, options
+      return link_to name, download_task_path(problem.id,basename,ext), target: '_blank'
     else
       return ''
     end
diff --git a/app/models/submission.rb b/app/models/submission.rb
--- a/app/models/submission.rb
+++ b/app/models/submission.rb
@@ -154,7 +154,7 @@
       return if self.user.admin?
 
       #check if user has the right to submit the problem
-      errors.add('problem',"must be valid.") if (!self.user.available_problems.include?(self.problem)) and (self.new_record?)
+      errors[:base] << "Authorization error: you have no right to submit to this problem" if (!self.user.available_problems.include?(self.problem)) and (self.new_record?)
     end
   end
 
diff --git a/app/models/user.rb b/app/models/user.rb
--- a/app/models/user.rb
+++ b/app/models/user.rb
@@ -83,7 +83,11 @@
   end
 
   def admin?
-    self.roles.where(name: 'admin').count > 0
+    has_role?('admin')
+  end
+
+  def has_role?(role)
+    self.roles.where(name: role).count > 0
   end
 
   def email_for_editing
@@ -275,9 +279,10 @@
     end
   end
 
+  # new feature, get list of available problem in all enabled group that the user belongs to
   def available_problems_in_group
     problem = []
-    self.groups.each do |group|
+    self.groups.where(enabled: true).each do |group|
       group.problems.where(available: true).each { |p| problem << p }
     end
     problem.uniq!
@@ -298,6 +303,8 @@
     end
   end
 
+  #check if the user has the right to view that problem
+  #this also consider group based problem policy
   def can_view_problem?(problem)
     return true if admin?
     return available_problems.include? problem
diff --git a/app/views/application/_submission.html.haml b/app/views/application/_submission.html.haml
--- a/app/views/application/_submission.html.haml
+++ b/app/views/application/_submission.html.haml
@@ -10,7 +10,7 @@
   %td
     = submission.source_filename
     = " (#{submission.language.pretty_name}) "
-    = link_to('[load]',{:action => 'source', :id => submission.id})
+    = link_to '[load]', download_submission_path(submission)
   %td
     - if submission.graded_at
       = "Graded at #{format_short_time(submission.graded_at)}." 
diff --git a/app/views/graders/list.html.haml b/app/views/graders/list.html.haml
--- a/app/views/graders/list.html.haml
+++ b/app/views/graders/list.html.haml
@@ -10,10 +10,10 @@
   .panel-heading
     Grader control:
   .panel-body
-    =link_to 'Start Graders in grading env', { action: 'start_grading'}, class: 'btn btn-default', method: 'post'
-    =link_to 'Start Graders in exam env', { action: 'start_exam'}, class: 'btn btn-default', method: 'post'
-    =link_to 'Stop all running Graders', { action: 'stop_all'}, class: 'btn btn-default', method: 'post'
-    =link_to 'Clear all data', { action: 'clear_all'}, class: 'btn btn-default', method: 'post'
+    =link_to 'Start Graders in grading env', { action: 'start_grading'}, class: 'btn btn-default'
+    =link_to 'Start Graders in exam env', { action: 'start_exam'}, class: 'btn btn-default'
+    =link_to 'Stop all running Graders', { action: 'stop_all'}, class: 'btn btn-default'
+    =link_to 'Clear all data', { action: 'clear_all'}, class: 'btn btn-default'
 
 .row
   .col-md-6
diff --git a/app/views/groups/_form.html.haml b/app/views/groups/_form.html.haml
--- a/app/views/groups/_form.html.haml
+++ b/app/views/groups/_form.html.haml
@@ -5,12 +5,23 @@
       %ul
         - @group.errors.full_messages.each do |msg|
           %li= msg
-
-  .form-group.field
-    = f.label :name
-    = f.text_field :name, class: 'form-control'
-  .form-group.field
-    = f.label :description
-    = f.text_field :description, class: 'form-control'
-  .form-group.actions
-    = f.submit 'Save', class: 'btn btn-primary'
+  .row
+    .col-md-6
+      .form-group.field
+        = f.label :name
+        = f.text_field :name, class: 'form-control'
+  .row
+    .col-md-6
+      .form-group.field
+        = f.label :description
+        = f.text_field :description, class: 'form-control'
+  .row
+    .col-md-6
+      .checkbox
+        = f.label :enabled do
+          = f.check_box :enabled
+          Enabled
+  .row
+    .col-md-6
+      .form-group.actions
+        = f.submit 'Save', class: 'btn btn-primary'
diff --git a/app/views/groups/index.html.haml b/app/views/groups/index.html.haml
--- a/app/views/groups/index.html.haml
+++ b/app/views/groups/index.html.haml
@@ -7,14 +7,16 @@
     %tr
       %th Name
       %th Description
+      %th Enabled?
       %th
       %th
 
   %tbody
     - @groups.each do |group|
-      %tr
+      %tr{:class => "#{(group.enabled?) ? "success" : "danger"}", id: "group-#{group.id}"}
         %td= group.name
         %td= group.description
+        %td= toggle_button(group.enabled?, toggle_group_path(group), "group-enabled-#{group.id}", size: ' ', block: ' ')
         %td= link_to 'View', group, class: 'btn btn-default'
         %td= link_to 'Destroy', group, :method => :delete, :data => { :confirm => 'Are you sure?' }, class: 'btn btn-danger'
 
diff --git a/app/views/groups/toggle.js.haml b/app/views/groups/toggle.js.haml
new file mode 100644
--- /dev/null
+++ b/app/views/groups/toggle.js.haml
@@ -0,0 +1,8 @@
+= render partial: 'toggle_button',
+                  locals: {button_id: "#group-enabled-#{@group.id}",button_on: @group.enabled }
+:plain
+  r = $("#group-#{@group.id}");
+  r.removeClass('success');
+  r.removeClass('danger');
+  r.addClass("#{@group.enabled? ? 'success' : 'danger'}");
+
diff --git a/app/views/layouts/_header.html.haml b/app/views/layouts/_header.html.haml
--- a/app/views/layouts/_header.html.haml
+++ b/app/views/layouts/_header.html.haml
@@ -68,7 +68,7 @@
                 = add_menu( 'Login Report', 'report', 'login')
             - if (ungraded = Submission.where('graded_at is null').where('submitted_at < ?', 1.minutes.ago).count) > 0
               =link_to "#{ungraded} backlogs!",
-                grader_list_path,
+                graders_list_path,
                 class: 'navbar-btn btn btn-default btn-warning', data: {toggle: 'tooltip'},title: 'Number of ungraded submission'
 
         %ul.nav.navbar-nav.navbar-right
diff --git a/app/views/main/help.html.haml b/app/views/main/help.html.haml
--- a/app/views/main/help.html.haml
+++ b/app/views/main/help.html.haml
@@ -43,5 +43,5 @@
           %tt <tt>{<br/>LANG: Pascal<br/>TASK: mobiles<br/>}</tt>
     
     %p
-      = raw(t('help.ask_questions_at_messages',:message_link_name => (t 'menu.messages'),:url => url_for(:controller => 'messages', :action => 'list')))
+      = raw(t('help.ask_questions_at_messages',:message_link_name => (t 'menu.messages'),url: messages_path ))
 
diff --git a/app/views/problems/stat.html.haml b/app/views/problems/stat.html.haml
--- a/app/views/problems/stat.html.haml
+++ b/app/views/problems/stat.html.haml
@@ -6,19 +6,23 @@
 %h1 Problem stat: #{@problem.name}
 %h2 Overview
 
+.row
+  .col-md-2
+    %strong Name:
+  .col-md-10
+    = @problem.full_name #in_place_editor_field :problem, :full_name, {}, :rows=>1
+    = link_to_description_if_any "[#{t 'main.problem_desc'}] <span class='glyphicon glyphicon-file'></span>".html_safe, @problem
+.row
+  .col-md-2.strong
+    %strong Submissions:
+  .col-md-10
+    = @submissions.count
+.row
+  .col-md-2.strong
+    %strong Solved/Attemped User
+  .col-md-10
+    #{@summary[:solve]}/#{@summary[:attempt]} (#{(@summary[:solve]*100.0/@summary[:attempt]).round(1)}%)
 
-%table.info
-  %thead
-    %tr.info-head
-      %th Stat
-      %th Value
-  %tbody
-    %tr{class: cycle('info-even','info-odd')}
-      %td Submissions
-      %td= @submissions.count
-    %tr{class: cycle('info-even','info-odd')}
-      %td Solved/Attempted User
-      %td #{@summary[:solve]}/#{@summary[:attempt]} (#{(@summary[:solve]*100.0/@summary[:attempt]).round(1)}%)
 
 %h2 Submissions Count
 = render partial: 'application/bar_graph', locals: { histogram: @histogram }
diff --git a/app/views/submissions/show.html.haml b/app/views/submissions/show.html.haml
--- a/app/views/submissions/show.html.haml
+++ b/app/views/submissions/show.html.haml
@@ -42,6 +42,7 @@
             - if @submission.problem!=nil
               = link_to "[#{@submission.problem.name}]", stat_problem_path(@submission.problem)
               = @submission.problem.full_name
+              = link_to_description_if_any "[download] <span class='glyphicon glyphicon-file'></span>".html_safe, @submission.problem
             - else
               = "(n/a)"
         %tr
diff --git a/app/views/user_admin/admin.html.haml b/app/views/user_admin/admin.html.haml
--- a/app/views/user_admin/admin.html.haml
+++ b/app/views/user_admin/admin.html.haml
@@ -1,25 +1,54 @@
-%h1 Administrators
+%h1 Modify Role
+.row
+  .col-md-6
+    %h4 Administrators
+    = form_tag modify_role_user_admin_index_path, method: 'post', class: 'form-inline' do
+      = hidden_field_tag :role, 'admin'
+      .form-group
+        = label_tag :login, 'Grant admin role to:'
+        = text_field_tag 'login',nil, class: 'form-control'
+      .form-group
+        = submit_tag 'Grant', class: 'btn btn-primary'
+    %br
+    %table.table.table-condense.table-hover.table-striped.table-bordered
+      %thead{:class => 'info-head'}
+        %th #
+        %th Login
+        %th Full name
+        %th 
+      - @admins.each_with_index do |user, i|
+        %tr
+          %td= i+1
+          %td= user.login
+          %td= user.full_name
+          %td
+            - if user.login!='root'
+              = link_to '[revoke]', modify_role_user_admin_index_path( login: user.login, role: 'admin', commit: 'revoke')
+  .col-md-6
+    %h4 Teacher Assistants (TA)
+    = form_tag modify_role_user_admin_index_path, method: 'post', class: 'form-inline' do
+      = hidden_field_tag :role, 'TA'
+      .form-group
+        = label_tag :login, 'Grant TA role to:'
+        = text_field_tag 'login',nil, class: 'form-control'
+      .form-group
+        = submit_tag 'Grant', class: 'btn btn-primary'
+    %br
+    %table.table.table-condense.table-hover.table-striped.table-bordered
+      %thead{:class => 'info-head'}
+        %th #
+        %th Login
+        %th Full name
+        %th 
+      - @tas.each_with_index do |user, i|
+        %tr
+          %td= i+1
+          %td= user.login
+          %td= user.full_name
+          %td
+            - if user.login!='root'
+              = link_to '[revoke]', modify_role_user_admin_index_path( login: user.login, role: 'TA', commit: 'revoke')
 
-%table{:class => 'info'}
-  %tr{:class => 'info-head'}
-    %th #
-    %th Login
-    %th Full name
-    %th 
-  - @admins.each_with_index do |user, i|
-    %tr
-      %td= i+1
-      %td= user.login
-      %td= user.full_name
-      %td
-        - if user.login!='root'
-          = link_to '[revoke]', :action => 'revoke_admin', :id => user.id
-%hr
-
-= form_tag :action => 'grant_admin' do
-  = label_tag :login, 'Grant admin permission to:'
-  = text_field_tag 'login',nil, class: 'input-field'
-  = submit_tag 'Grant', class: 'btn btn-primary'
 
 %hr/
 = link_to '[go back to index]', :action => 'index'
diff --git a/app/views/user_admin/new_list.html.haml b/app/views/user_admin/new_list.html.haml
--- a/app/views/user_admin/new_list.html.haml
+++ b/app/views/user_admin/new_list.html.haml
@@ -26,6 +26,15 @@
               is empty, the original value will be used instead.
             %li
               If the users with the same user_id already exists, existing information will be overwritten.
+          Example:
+          %ol
+            %li 
+              %pre user1,Somchai Jaidee
+              will create (or update) a user with login "user1" and setting the fullname to "Somchai Jaidee", also setting a random password.
+            %li 
+              %pre user1,Somchai Jaidee,
+              will create (or update) a user with login "user1" and and setting the fullname "Somchai Jaidee". No change is made to the password unless this is a new user. If this is a new user, a random password will be generated.
+
 
   .row
     .col-md-6
diff --git a/config/routes.rb b/config/routes.rb
--- a/config/routes.rb
+++ b/config/routes.rb
@@ -54,6 +54,7 @@
       post 'add_problem', to: 'groups#add_problem', as: 'add_problem'
       delete 'remove_problem/:problem_id', to: 'groups#remove_problem', as: 'remove_problem'
       delete 'remove_all_problem', to: 'groups#remove_all_problem', as: 'remove_all_problem'
+      get 'toggle'
     end
     collection do
 
@@ -92,7 +93,6 @@
       get 'download'
       get 'compiler_msg'
       get 'rejudge'
-      get 'source'
     end
     collection do
       get 'prob/:problem_id', to: 'submissions#index', as: 'problem'
@@ -113,8 +113,7 @@
       get 'admin'
       get 'active'
       get 'mass_mailing'
-      get 'revoke_admin'
-      post 'grant_admin'
+      match 'modify_role', via: [:get, :post]
       match 'create_from_list', via: [:get, :post]
       match 'random_all_passwords', via: [:get, :post]
     end
@@ -184,22 +183,23 @@
 
   #
   get 'tasks/view/:file.:ext' => 'tasks#view'
-  get 'tasks/download/:id/:file.:ext' => 'tasks#download'
+  get 'tasks/download/:id/:file.:ext' => 'tasks#download', as: 'download_task'
   get 'heartbeat/:id/edit' => 'heartbeat#edit'
 
   #grader
-  get 'graders/list', to: 'graders#list', as: 'grader_list'
+  #get 'graders/list', to: 'graders#list', as: 'grader_list'
   namespace :graders do
     get 'task/:id/:type', action: 'task', as: 'task'
     get 'view/:id/:type', action: 'view', as: 'view'
     get 'clear/:id', action: 'clear', as: 'clear'
-    get 'stop'
-    get 'stop_all'
-    get 'clear_all'
-    get 'clear_terminated'
     get 'start_grading'
     get 'start_exam'
+    get 'clear_all'
+    get 'stop_all'
 
+    get 'stop'
+    get 'clear_terminated'
+    get 'list'
   end
 
 
diff --git a/db/migrate/20200813083020_add_enabled_to_group.rb b/db/migrate/20200813083020_add_enabled_to_group.rb
new file mode 100644
--- /dev/null
+++ b/db/migrate/20200813083020_add_enabled_to_group.rb
@@ -0,0 +1,5 @@
+class AddEnabledToGroup < ActiveRecord::Migration[5.2]
+  def change
+    add_column :groups, :enabled, :boolean, default: true
+  end
+end
diff --git a/db/seeds.rb b/db/seeds.rb
--- a/db/seeds.rb
+++ b/db/seeds.rb
@@ -225,6 +225,7 @@
 end
 
 def seed_roles
+  Role.find_or_create_by(name: 'TA')
   return if Role.find_by_name('admin')
 
   role = Role.create(:name => 'admin')