The contract and advertisement states that you will teach 16 lessons per week. Upon arrival that will change to 24 lessons per week as there are never enough foreign teachers working there. You will be offered 78 rmb for 16 45 minute lessons as per the contract. What is not in the contract is that for the extra 8 to 10 lessons you will be offered just 30 rmb.

So, did you try to negotiate with them about their attempted imposition of sth not as per contract? Did you also point out you didn't go there for overtime, not even at 100 rmb/lesson? I'd have wiped the floor with them, and approached the issue with my cellphone switched on on record mode. Folks, though contracts mention overtime, you're not obliged to work it. I for one turn it down on principle every time.
Bad start by succumbing to their unreasonable offer means they'll take the crap even worse thereafter. But the trouble with Guangdong is they csn replace u too easily there. Head north, where their counterparts' options are severely limited. Too many foreign weasels where u are!