I don't understand, how exactly do they control your visa? Do they have your passport, because that's where your visa is, or should be. Do you mean your residence permit? What exactly are you talking about, be specific?

Now would certainly be a bad time to go, however, if they have broken contract terms you have every right to leave, and get your visa back. The truth is schools like this just want to make it very hard for you to leave. In the end though, if your determined, they will have no choice but to return whatever they are holding that is yours.

Schools do this,(even though in the end they can't win), for the sole purpose of discouraging current, as well as furture teachers from leaving. They can't actually keep your stuff, but if your not man enough to get your things, and make a fuss, than certainly they will. You can easily call the PSB, make a stink, and in the end they will return your things. You can try being loud, or just so persistent as to not leave until they give you what is yours. It works, believe me, but you must put your foot down. Basically your letting them walk all over you.

I have zero sympathy for you though, you said you were well aware that they might be bad, and chose voluntarily not to research them. No offense, but that makes you a fool. Why have a bad time here just because it's close to your father? Wuhan has a lot of schools, many much better than this outfit. You really only have yourself to blame, you say you knew better but did worse anyhow. Blame for ending up there lies with you, blame for being a crappy school lies with them. Case closed.