Personally, if it's only 9 dollars an hour, I would say you can easily find another job that will pay that much or more. If it's affecting your grades, that can likewise affect your chances of getting better jobs in the future, right? I mean... your grades can HELP you get that better job or position... don't you think?
This being said, can you possibly ask a supervisor to lessen your hours for the next few weeks? Is that a possibility? That may give you the timet hat you need for studying and the like, you know? As well... have you ever considered taking a train or bus to work? It may take a bit longer, but this would give you time to study while travelling. I've had a few friends that had an hour commute to school and the like... that time could be used towards studying :), whereas a car takes away that option.
So personally, I think it's up to you. See if you can either reduce your hours, or possibly commute to work. Or maybe take time off (That would be hard to do without notice). If not, go for the schooling... but start looking for other jobs immediately as well, especially when your bills rely on it, you know? Personally, if you could find another job or reduce your hours BEFORE leaving this one, that would be ideal. You don't want to be stuck in a situation where you can't afford your lifestyle.