How to you ask to be demoted?

I agree, talk to your manager before composing a letter. Are you sure you can't work 24 hours a week and go to school? Check out your school schedule and see if you can work the cles so they're Tuesday/Thursday or Mon/Wed/Fri. This way, you can give full days when you're available and those when you're not. Don't forget to consider the needs of the hospital; if Friday night is really busy, only schedule cles Friday morning with enough time to come in and work a shift that night.