That seems ridiculous. Do you have a Z visa? Contracts between foreign teacher and school tend to have a different meaning in Asia. In the west a contract is a binding agreement that is subject to change only after mutual agreement. In Asia a single party (but not you, lol) may change a contract and you are expected to go along with it. Normally its not a big deal but if they are having you teach in a different school, different age groups, and expect you to have lesson plans for the next 6 months for each student, I would say its probably best to just leave unless they pay you accordingly. Sounds like they have a lot of issues, its going to be a tough fight to make those demands reasonable.

Do you have a Z visa? If not its very easy for you to leave bad employers behind because they can't pursue you as they are breaking Chinese law. If you have a Z visa, its going to be a little more difficult and I can't offer any advise on it.