No, you won't be stopped at customs, this is not the police state people in the west seem to think it is. I mean, in some ways it is, but not this one.

Consequently, if your school lied than you can leave, because it means the broke the contract, yes? Whatever the case, they can't do anything whatsoever. Trust me on this, just leave, nothing at all will happen.

The school could try and get you blacklisted, which would make finding legal work in China in the future difficult, however it may only apply to that particular province.

You're free to leave whenever you like, and no one will stop you, and nothing bad will happen, just don't tell the school. Might be wise to leave at say midnight, and if your plane don't leave until the next day, get a motel room somewhere. Don't leave during the day though, schools have been known, on rare occasion, to try and stop a teacher.

There you go, all you need to know, well, maybe not all, all being a broad word, but certainly enough. Get the hell out of dodge my boy, you'll be fine ;)