Thought it's something that's becoming common, I'd have to say it concerns me that people would work on a student visa, you simply have no protections whatsoever under the law. There is little enough protection as it is, take that away and you are really in trouble.

Only work for a school that is valid and legal. Chinese bosses here bank on a foreigners ignorance, as well a foreigners ideas about how contracts work based upon their own experience in western countries. To us, a contract is legal once signed, in China, that is not the case. Here contracts don't mean much, and for a foreigner, are only legal in regards to employment if you are legally allowed to work here. Meaning if you have the proper work permit. If you do not, you can sign all the contracts you want in triplicate, it won't make any difference, the contract is useless. Chinese bosses know that most Americans have these hold over ideas from their own culture, and they bank on this fact.

More and more Chinese bosses try to get foreigners to sign contracts even if they are only working part time for a company. Though most of us know this is nonsense, and that any contract signed under such circumstances is not legal, many foreigners won't know that, but the boss does. He will in fact bank on your own adherence to your holdover beliefs about contracts to make you more loyal and bound to his company, or else you'll face consequences if you breach our contract. Of course the contract is worthless, again, your ignorance plays into their hands. Meanwhile they can feel free to take advantage and use you to their own gain.

Contract law is much more simple here in China, at least for us. If you are not legally employed by that company and that company alone, any contracts is null and void for both parties, and the equivalent of a handshake.