In Grid view how to add another one field?

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ASP.NET

here is my aspx code:

<asp:GridView ID="GridViewStudentsList" runat="server" AutoGenerateColumns="False"
               BackColor="White" BorderColor="White" BorderStyle="Ridge" BorderWidth="2px" CellPadding="3"
               CellSpacing="1" EmptyDataText="No Details Found" GridLines="None" OnPageIndexChanging="GridViewStudentsList_PageIndexChanging"
               PagerStyle-Font-Bold="true" PagerStyle-ForeColor="#000066" PageSize="5" Style="z-index: 101;
               left: 60px; position: absolute; top: 80px">
               <RowStyle BackColor="#DEDFDE" ForeColor="Black" />
               <FooterStyle BackColor="#C6C3C6" ForeColor="Black" />
               <PagerStyle BackColor="#C6C3C6" ForeColor="Black" HorizontalAlign="Right" />
               <SelectedRowStyle BackColor="#9471DE" Font-Bold="True" ForeColor="White" />
               <HeaderStyle BackColor="#000066" Font-Bold="True" ForeColor="#E7E7FF" />
               <Columns>
                   <asp:BoundField DataField="Sno" HeaderText="Sno" />
                   <asp:BoundField DataField="RollNo" HeaderText="RollNo" />
                   <asp:BoundField DataField="Name" HeaderText="Name" />
                   <asp:BoundField DataField ="Department" HeaderText ="Department" />
                   <asp:BoundField DataField ="Year" HeaderText ="Year" />

               </Columns>


           </asp:GridView>

And this is my aspx.cs:

protected void GridViewStudentsList_PageIndexChanging(object sender, GridViewPageEventArgs e)
    {
        try
        {
            GridViewStudentsList.PageIndex = e.NewPageIndex;
            FillStudentList();
        }
        catch (Exception ex)
        {
            throw ex;
        }
    }

public void FillStudentList()
  {
      try
      {
          SqlAdapter = new SqlDataAdapter("select ''Sno,RollNo,Name,Department,StudentsAccounts.Year from StudentsAccounts inner join(select teacherid,Departmentid,dptName,year from TeachersWorkList inner join Departments on Departments.dptid=TeachersWorkList.departmentid Where Teacherid='" + HiddenFieldID.Value + "')As DptList On StudentsAccounts.Department=DptList.dptName And StudentsAccounts.Year=DptList.year where DptList.Departmentid='" + Session["MyDptID"].ToString() + "' And StudentsAccounts.Year='" + Session["Year"].ToString() + "' ", Connection);
          DataSet MyDataSetDetails = new DataSet();
          SqlAdapter.Fill(MyDataSetDetails, "TeachersWorkList");
          for (int i = 0; i <= MyDataSetDetails.Tables[0].Rows.Count - 1; i++)
          {
              MyDataSetDetails.Tables[0].Rows[i][0] = i + 1;
          }
          GridViewStudentsList.DataSource = MyDataSetDetails;
          GridViewStudentsList.DataBind();
      }
      catch (Exception ex)
      {
          throw ex;
      }
  }

Now i want to add a extra field mark..where i add a mark?? also in this grid view how can i provide marks for each student? anyone can help me??