When your SM returns, let them know about the scheduling issue since they are the ones who make the schedule.

In the meantime, try to find someone to cover for you.

If you can’t find someone, let the shift supervisor know ahead of time that you can’t come in due to school (they’ll most likely be cool with it, I’ve done this before). Worse things worse is the shift supervisor will try to push back about you missing the shift, but your school is a priority! Don’t let anyone try to tell you this job is more important than school/personal life.

Text your sm. Assuming you put in your availability, thats their problem

When my manager takes a vacation, they ask another manager to watch the store. If you have a point of contact, I would bring it up.

Try to get it covered and if you can’t I’d just let a supervisor know they’ll understand

I always get confused when managers do it because you can’t do it by accident. When you try to schedule outside availability, the system flags it and you have to self override. Text your SM and let the shift supervisor know. Worse thing you’ll have to do is call out but try to get it covered if she doesn’t change it.

It is almost certainly a mistake, probably from copying a previous week's schedule before your school availability.
Your first and most effective course of action is to try to get the shift/s covered and then have a talk with the SM who is proxying your store about what other help they can provide.

Your SM doesn't have to follow your availability, though they are certainly supposed to try to. Schedules are completed based on "needs of the business" first. It absolutely sucks, but it's the policy in the handbook, so in terms of covering your ass, treat the shift/s as though it's yours and do your due diligence in getting coverage.
I'm sorry!