It depends on whether your employer pisses anyone off.  If they don't pay the right bribes, you will get raided by police.  And when that happens you will be hit with a fine and have your visa revoked for working illegally.  You will lose any tuition you paid to your school, advance rent you paid for your apartment, etc...  Is the extra money really worth giving up everything else?

You never know, do you?

They will sometimes check for illegal teachers, working without the correct documents. China is cracking down on illegal and unqualified teachers, as well as the schools and agents which employ them.

Or the school may just decide to get rid of you at the end of the contract without paying you, and tip off the visa police that you are working illegally.

if you break the law just be prepared for a 10 year ban