loop problem,PLEASE HELP!!!!!!!!

Try it like this:

    static void Main()
    {
       int marks;
       string name;

       for(int student = 1; student <= 8; student++)
       {
          Console.WriteLine();
          Console.WriteLine("Enter details for student " + student);
          Console.WriteLine();
          Console.Write(" Name  : ");
          name = Console.ReadLine();
          Console.Write(" Marks : ");
          marks = int.Parse(Console.ReadLine());
          if (marks < 0 || marks > 100)
          {
             Console.WriteLine();
             Console.WriteLine("Error : marks must be between 0 and 100");
             return;
          }
 
          switch(marks/20)
          {
              case 2:
                Console.WriteLine(" Grade : Pass");
                break;
              case 3:
                Console.WriteLine(" Grade : Merit");
                break;
              case 4:
              case 5: 
                Console.WriteLine(" Grade : Distinction");
                break;
              default:
                Console.WriteLine(" Grade : Fail");
                break;
           }
        }

        Console.WriteLine();
        Console.Write("Press enter to end program");
        Console.ReadLine();
     }