As I am unable to read your language, I'm not sure which table occurrence is which.
My best guess is that you want to be able to select Subjects for a class by using a check box format so that the same subject cannot be selected more than once.
If this is what you want, I have a demo file where a check box format is used to select contacts for an event using the same many to many relationship that you have defined in your database. It's in an older file format so if you are using FileMaker 12 or newer, use Open from FileMaker's File menu to open this file and produce a copy converted to the newer file format: https://www.dropbox.com/s/oyir7cs0yxmbn6i/ManyToManywDemoWExtras.fp7
Thank you for your answer and for you support. I will try your demo file.