What is Wrong so Fustrated???

using System.Collections.Generic;

namespace Treehouse.CodeChallenges
{
    public class Course
    {
        public Course()
        {
            CourseStudent Students = new List<CourseStudent>();
        }

        public int Id { get; set; }
        public int TeacherId { get; set; }
        public string Title { get; set; }
        public string Description { get; set; }
        public int Length { get; set; }

        public Teacher Teacher { get; set; }
        public ICollection<Student> Students { get; set; }
    }
}

using System.Collections.Generic;

namespace Treehouse.CodeChallenges
{
    public class CourseStudent
    {
        public int Id { get; set; }
        public int CourseId { get; set; }
        public int StudentId { get; set; }
        public decimal Grade { get; set; }

        public Course Course { get; set; }
        public Student Student { get; set; }
    }
}

using System.Collections.Generic;

namespace Treehouse.CodeChallenges
{
    public class Student
    {
        public Student()
        {
            Courses = new List<Course>();
        }

        public int Id { get; set; }
        public string FirstName { get; set; }
        public string LastName { get; set; }

        public ICollection<Course> Courses { get; set; }
    }
}