My advice is to stay away from this job. Truthfully, I know nothing about the school. But I can tell you that Dongguan is one of the most polluted places in China. Seriously! You can't see the sun there due to the pollution, and in the morning you can see a fine dusting of pollutants on the black cars.I spent a week there and vowed never to return.
Furthermore, the salary of 4000RMB is a joke for anywhere in China these days, but will get you nowhere in Dongguan considering the cost of living there.
Accept this job at your own risk.