Wow, your headmaster sounds like a real jerk. I had a similar problem with the curfew thing when I first lived in China, however, we were able to get them to drop it, throung veiled threats of us leaving and/or being most unhappy and spreading bad press about them so that they would never be able to find new foreign teachers. Curfew was never mentioned again.

However, in your case it sounds like they might stick to their guns, in which case read your contract again carefully. Is there any mention of a penalty that you would have to pay if you broke the contract? (The curfew, though not in the contract, is considered to be a "verbal contract" meaning that if you stay out past curfew too many times they may consider inacting some kind of disciplinary action, in most cases it involves money.) Anyway, if there is somekind of penalty for breaking the contract, I suggest lying. Yes it's a sin, but a good sob story can be most effective, death of a beloved relative or some kind a fatal illness works well. Just make sure you've lined up another job first.

I know this might sound a bit harsh, but if you don't feel comfortable and happy where you're working and living, your time in China is going to suck. China can get really mental in both the good and bad sense of the word, and you need a little space of your own where you feel somewhat normal. Hopefully that made some sense. Anyway, good luck and I hope that you are successful in getting the curfew lifted.