I've been one so here let me just tell you. You have to call up your coworkers on their days off and beg them to cover you. You promise or call in favors, you convince someone to give up time with their family or skip class or to work a double shift for you. If you can't get anyone to do it you work sick.

If you can't get it covered and actually don't go to work the manager will work the shift and hold it against you forever. They can fire you for it the first time and they really might, it all depends on your relationship with them. At the very least your relationship, whatever it was, is badly damaged. They make the schedule so I hope you don't need consistent hours, or a specific day off for class or anything.