Which Model

[eluser]rogierb[/eluser]
Which courses does a student attend => student model => from point of student
Which students are in a course => course model => from point of course
Enroll a student in a course => course model => from point of course
A student wants to enroll in a course =>student model => from point of student

Or create a student_course model to house everything that has to do with both

To be honest, it doesnt really matter that much in this case. As long as you are comfortable with it.