Do you have the contact information of anyone in the university's leadership? I might recommend reasoning with one of them. There are a number of possibilities for this situation:

--The head teacher is acting alone. He reported one thing to management (that the grades should be his way), and now there is fly in the ointment. He does not want to embarrass himself by reporting your numbers, and he is not telling the management that he is bullying you to change the grades. If you keep on making noises indicating that this will become an even larger issue...he will fold. This is classic, classic China.

--The school is trying to stiff you on your summer pay. I would find the SAFEA office in charge of your province. Send them a scan of your contract, the letters of recommendation and release, and his messages. It could be that the SAFEA employees are too lazy to do anything (unfortunately, I have found it that you have to stand over Chinese people to make them do their jobs sometimes), or that the uni has guanxi with them...but it might work.

--The school actually thinks that you should change your grades. In which case, they should have made noises before you flew home. I refer you to my earlier advise.