Not to give you any release papers is common practice in China. They consider you an investment on their part and don't want to make it easy for you to work for one of their competitors. In other words, you are one of their slaves. The best thing that could happen to you would be not to work at all for such "schools" and not waste your time and energy on them. They will never respect you.

" If you want to stay in China after finishing this contract with the school you cannot get legal work as they won't give you these release papers and you will be stuck".