Nico, here is an example domain model from a sample project with Students and classes. This may help in your overall design:
You may have to create some select pages that use Microflow buttons to link multiple students to a course.
First decide (or let the "customer" decide) what the process is.
Add students to a class or assign a class to a student. Because this determines the Owner of the association/where association can be set.
Then ask yourself, does a student attend a single class or multiple classes (a year/through whole school). (my guess, in the end student will attend multiple classes)
When adding students to a class; owner (dot) of the association is Class + *-* association.