Otherwise it is the responsibility of the custodial staff who maintain the rest of the school grounds to clean the cafeteria; that is what they are paid for and since you are not compensated in any way for your time, it is not only wrong and downright lazy to not hire a new employee, but I would think it is techically illegal (At least in the U.S.) to have you perform work intended for a paid employee of the school district and not pay or otherwise compensate you for your time.
Now that I think about, I have indeed seen other people around the school, mopping the floors and steps and such. Get them to do it. There are a couple of exchange students that have to do odd jobs around the school to help pay for the dorms and their way through school, etc. Not to sound rude or racist, but why don't they change of their schedules to do this?
Realistically I would consider talking to your parents about this and assuming they agree that you are being taken advantage of, first I would sit down with the principal and discuss your concerns, and then if he continues to be "unavailable" or unwilling to change his position then perhaps talk to the school district superintendent and request that something be done about this issue.
Well, I have talked to my parents, and they admit that it sounds more like punishment. Also, next week is my spring break, so I've got plenty of time to think about what to say to the principal. All I know is, something has to be done. But here's the thing: I go to a private school, so no superintendent. My school has preschool/kindergarten through college all on the same campus, and the academy (preschool/kindergarten through high school) principal has basically the final say on academy issues. There is a president of the campus, as well as a vice president, and a board, but they don't bother with the academy, just the college. I don't think that talking to the president would help anyway. The principal knows pretty much everybody in the academy, but the president and his board only know a few academy students, and those are the high schoolers that do alot with the college, like take alot of college classes. Pfft.
Oh well.