no of course not!! your education comes first! just be sure to be polite about it and maybe ask some of your coworkers to take your shifts?

since its next week you should find coverage for it or tell your manager which shift you won't be able to make so they can look for coverage as well. you should let them know ASAP that you can't work more than 20 hours so they won't keep scheduling you. some people are understanding but it could get annoying to have to keep finding coverage for your shifts if you don't let them know as soon as possible.

if it were me i'd just talk to coworkers and see if i could find someone to cover my shift!