public async Task <IActionResult> GradeStudent(int id, string studentId, Grade grade)
        {
            if (studentId is null)
            {
                return(BadRequest());
            }

            var userId = userManger.GetUserId(User);

            if (!await trainers.IsTrainer(id, userId))
            {
                return(BadRequest());
            }

            var success = await trainers.AddGrade(id, studentId, grade);

            if (!success)
            {
                return(BadRequest());
            }

            return(RedirectToAction(nameof(Students), new { id }));
        }