The signed contract is meaningless. It is not actually signed (stamped Chinese style) until you arrive and sometimes not until your residency permit is approved.

If you actually have a 'Z' visa though, you have been entered into the system as receiving the visa in the name of the school that sponsored the visa.

In that case you need to send a letter TO THE SCHOOL (not the recruiter) saying you are not able to travel to China now because ___________ (enter a lie to give them some face; lies about family crises work best). Ask them to please forward you a 'release letter'.

This will work for any position starting in August/September. You will not be able to get another Z visa until the release is made by the school, so you are unable to work 'legally' for the current semester.

If your visa is a tourist or business visa, you would have been working illegally anyway. If that is the case, you can immediately choose another employer that provides you a Z visa. The L or F visa will be cancelled by the Embassy/consulate when they process the Z visa.