Get a dialogue going between your boss, and university. Make them sort it out.

Work for the 28 hours, then volunteer for the remaining 12.

Actually, if you exceed your work hours because your boss demands it, it is he that is in trouble with the university, not you.

There must be someone you can discuss this will, the liaison between the University and your work place. Perhaps your boss does not know University policy but the person at the University that is giving you credit for the job would.

So you go to the university's Employment Services and/or Academic Affairs office. Your boss can't fire you for refusing to break campus policy.

Notify the university. If you're only allowed to work 28 hours per week, do the 28, and only the 28. your boss can figure out the next shift.

I'm guessing some of you haven't had bad bosses? There are many out there who would just fire someone without attempting to make any accommodation. Many small businesses don't have some HR department to answer to, and just shaft people as they wish.

this is student employment, which at least to me implied it's employment by the University. that means there is DEFINITELY an HR department to work with.

As true as that may be, it's clearly NOT the case here. Universities hire students to work in many offices on campus. The student workers' allowable hours are set by the university (at my college its 20 hours per week). The boss in this situation would be aware of the limitations and is required to abide by them.

Unless your boss is the owner of the company, there is someone who is higher up. Go to that person and explain the situation.

threaten with the labour board can't fire you then

Tell your boss what the university says and tell her that if she wants you to work longer, then she need to take it up with them.

welcome to the real world where you work overtime and lie about it