使用 GroupBy 选择 Max()，使用 linq 方法语法，EF

我有一堆带有一些数据的类：

public class Teacher
{
    public int Id { get; set; }
    public string Name { get; set; }
    public DateTime bDate { get; set; }
    //One-to-one with course

    public int CourseId { get; set; }
    public virtual Course Course { get; set; }
}


public class Course
{
    public int Id { get; set; }
    public string Name { get; set; }
    public virtual ICollection <Student_Course> Student_Courses { get; set; }
    public int TeacherId { get; set; }
    public virtual Teacher Teacher { get; set; }
}


public class Grade
{
    public int Id { get; set; }
    public int Mark { get; set; }
    //one-to-many with Student_Course

    public int Student_CourseId { get; set; }
    public Student_Course Student_Course { get; set; }
}

public class Student_Course
{
    public int Id { get; set; }
    //many-to-many with student
    public int StudentId { get; set; }
    public virtual Student Student { get; set; }

    //many-to-many with course
    public int CourseId { get; set; }
    public virtual Course Course { get; set; }
}

 public class Student
{
    public int Id { get; set; }
    public string Name { get; set; }
    public DateTime bDate { get; set; }
    //one-to-many with student_course
    public virtual ICollection <Student_Course> Student_Courses { get; set; }

}

现在我尝试编写一个查询 - 对于每个老师（姓名）都需要通过学生成绩输出他最好的学生（姓名），以及最好的成绩，从班级成绩，我的代码：

public IActionResult Task9()
    {
        var task9 = Db.Grades.GroupBy(gr => gr.Student_Course.Course.Teacher.Name).Select(x => new Task9
        {
            NameOfSt = x.Select(st => st.Student_Course.Student.Name).FirstOrDefault(),//Needs to correct this line
            NameOfTeac = x.Select(r => r.Student_Course.Course.Teacher.Name).FirstOrDefault(),
            BestMark = x.Max(gr => gr.Mark)


            //NameOfSt = Db.Students.FirstOrDefault(st => st.Id ==x.FirstOrDefault().Student_Course.StudentId).Name
        });
        return View(task9);
    }

以便代码输出正确的教师姓名和学生对课程的最佳分数。但学生名实际上是错误的。我该如何解决？谢谢你的帮助。

Teac BestMark NameStud