maybe you could just say "these are the days I can work. If you schedule me for any other days I will not be coming in" and follow through. I know they've tried to get you to work before because "no one else could come in", but fuck them. And if you feel like its interfering with your school work than its really time to rethink priorities. People go to college so they can make more than $9.50 someday. Its in your best interest to make that day sooner rather than later.