One thing to check in making your decision is whether there has been a "state of emergency" declared. This decision is usually made by the governor, and is different than a "snow emergency". If there is a state of emergency, it is against the law to be on the roads except for emergencies unless you are a first responder (or sometimes goverment worker, depending on the state).
If this is the case in ND or MN, the school cannot legally ask anyone who doesn't live on campus to be at rehearsal. This also means that your grade can't be affected by not going to rehearsal.
As for whether you should try to go anyway, there are several different factors to consider. Your personal safety is first and foremost. No show or grade is worth your safety. The wellbeing of your colleagues (cast/crew/etc) is also something to think about. As a student, you won't have a lot of success telling a department head what he should or shouldn't do, but you can give him factual information that may help him make a good decision. Info like how many hours it will take to restage scenes with anyone who couldn't attend the first time, the weather forecast for more snow/rain, what the city and state goverments are recommending for peoples safety, who would or would not be available (performers/choreographer/crew) if he did choose to have rehearsals without you.
If you give him this info and he still chooses to have rehearsal, trust your judgement. Although we frequently feel the opposite, the world will not end if we don't make it to rehearsal, especially in times of emergency. Document your conversations and your reasons for not being able to be there, know that you will probably have to work twice as hard when you return, and be available as much as possible by phone or email. That is the most you can do. Theatre has survived fires, floods, earthquakes, and every other natural disaster possible, and it is not worth you putting yourself in danger. Think of it as "the show will go on" versus "the show must go on".
Good luck with this very difficult decision! (and welcome to the boards!)
SSM