I have had some very brief experience with Chinese High School students and the school administration myself. Some of the administration people seem to think that foreign teachers who are adults and have a right to go in and out in their free time as they see fit are like their students whom they can order around and make them "obey" as it is expecxted from students. If you cannot cope with that, I'd rather suggest you leave them alone and teach adults instead.

The other thing is there are occasionally problems in discipline and behaviour, especially at private-run High Schools with a bunch of small princes and princesses who think they can do everything because their parents pay a lot of money to the school. In case of problems, do not try to punish them and report about that to the headmaster immediately. You can insist that you will not work in a class with special students present who are constant trouble-makers because of their behaviour. Have students stand up and salute you when you enter the class, then salute back and give them a sign that they can sit down. It sounds a bit odd to do this but it will be clear that you are the boss, and you will certainly have less problems.

Finally, what are you supposed to teach? Oral English? If so, and you have no textbook provided (there seem to be none prescribed by the competent authorities), collect materials freely available from the internet. First thing is make them speak, they are usually shy. Structure your lesson in a way that they are active most of the time themselves instead of you speaking all the time. Reduce your own role to that of a coach who lets them do most of the things themselves and who would only intervene when absolutely necessary.

Hope that helps.