I've got some problems with my job?? help me!?

I think that you took the job knowing that you'd have a restricted schedule when you went back to school. It's pretty much understood that the low man on the totem pole get the most undesirable shifts- weekend closing in this case. You are the low man. Either take the shifts, because it's what's necessary to keep the job, or get another job. Yes, you're a senior. But it's only high school. Life gets way better after high school.