Setting up tables for grades

I do not understand why I am not understanding this, but I am trying to set
up a form for inputing student grades. There are 13 classes that I need to
track grades for (currently) as well as a composite grade. I have a table
(tblStudents) with the student information (including [StudentID]), a table
(tblCourses) with course names(two fields, one [CourseID], the other
[Course]), and a table (tblCourseGrades) to keep track of the grades
([GradeID], [StudentID], [CourseID], [Grade]). I'm thinking I'm
understanding relational design and I have the table set-up correct, but
designing the form for inputing the grades I'm just not getting for some
reason or another. I have a combo box that selects the student, and then a
subform (datasheet view) with tblCourseGrades as its source. This is not
working. The subform only shows one blank row. Any help to straighten out
my understanding would be appreciated. I've tried reading various tutorials
on relational design and normalizing, but there is something that I'm just
not getting. Thanks in advance.