How do I politely tell my boss she's scheduled me too much next week?

Ask her to sit down with you when she has a moment, and just explain to her that you need extra time for school; basically what you've written here would be a good thing to tell her, it sounds polite, and doesn't come across as whiny. If you know that from now on, your school work is going to be a little more intense, this would also be a good opportunity to discuss you work schedule for the future, as it would be easier for her to give you the time off in the first place, rather than change the schedule after the fact.