Am sort of sad cause i would love to go China. Now they send me an invitation letter and work permit, and want me to get my Z-visa and leave asap. Can i look for others schools, or is it too late once they issue you those papers? I have the documents in my place, and have not applied for the Z-visa yet.

A bit of a no-brainer in my view.
You are morally obliged to take up the job offer.
If you wish more detailed advice on what to do when you get there, and on the attractiveness or otherwise of the whole contract you've signed, and the cost of living there, and house rental norms there, you should provide us, the other readers besides yourself, with your e-mail address and/or your Skype contact details.
Actually, I have taught in Changchun for one semester, so I have background information that would definitely be useful to you.

Your post is extremely relevant and accurate. Changchun Daqiao is nto an ethical employer. Best to be avoided.

Once the contract has been signed by both parties, the FT will be able to get his (or her) Residence Permit provided he (or she) has had a recent satisfactory medical exam result and can also submit his (or her) Foreign Expert Certificate to the local PSB Bureau at a point in time within 30 days of the date on which the Z visa has become valid. The FT may also need a satisfactory criminal check from his (or her) homeland, depending on the province in which the employment takes place. Some provinces do not require this.The RP will replace the Z visa. Any probationary period must be invalidated by the RP as the contract will include a clause allowing for the FT to be given some time to amend his teaching in the event of the employer pointing out failings in it.
In my view, probation can result in instant dismissal, whereas the 'failings' clause can allow the FT a stab at improving his (or her) unsatisfactory teaching. I reckon that if the 'failings' clause is invoked, the FT will be more than likely to be fired a week thereafter.