public ActionResult RemoveStudentFromGroup(RemoveStudentFromGroup model)
        {
            //complete operation
            GroupService service = new GroupService();
            service.RemoveUserFromGroup(db, model.StudentId);

            return RedirectToAction("Index","Group");
        }
        public ActionResult RemoveStudentFromGroup(int id)
        {
            Student student = db.Students.Find(id);

            if(student == null)
            {
                return HttpNotFound();
            }

            RemoveStudentFromGroup model = new RemoveStudentFromGroup()
            {
                StudentId = id,
                Student = student
            };
            return View(model);
        }