Exceed 20 hours per week as a working student?
If you work more than 20 hours per week as a working student, you are no longer in the "working student status".
Is that a problem? So I have to prove to some authority or something that I work 20 hours a week?
ne vice versa, you have to prove that you do not work more than 20 hours because otherwise it is assumed that you don't have enough time to study. This is the legal requirement.
It's simple – you're just gonna keep all the social insurance contributions from the pay office.