I can understand your anger only too well. I have worked for them as well with too long hours and very low pay considering the amount of hours I had been working for them.
They are agents, and not only do they take half your salary paid by the school for to you to them, they will also be slow in paying you the other half, will either delay it for some reasons or find fictitious reasons to deduct some horrendous amounts from your salary. They do that because they also always short of money - the large amounts of money they get from the schools is being used for speculation on the stock exchange market or for some other investment business. This is the way they operate, they are irresponsible people and make people coming to China under false pretences.

Do not trust these dishonest agents, do a runner if you work for them, and even better: DO NOT WORK FOR THESE CLOWNS AT ALL!