If you want to get American dollars regularly and not pay any commission then here is an idea. If the school does it legally then they only can do about 30% or so. If you go in the alley you don't know if the money is fake for us non Americans it is hard for us to tell. It is usually safe and money is real but you're going to pay a commission. I have discovered a better way that is somewhat legal and pay a flat rate.

Get one of your Chinese colleagues that you can trust with an ID card.

Then go to the largest Bank of China in town.

Give them the money that should be at least 1000 USD worth of Yuan.

They will fill out forms.

Then the bank will give them the USD.

After that take them out for a meal.

I don't know if it is 100% legal but technically the Chinese person is buying the USD but I wouldn't bring too much attention to yourself. So each time use a different colleague to go to the bank. The government doesn't want to see so much money leave the country so you're not doing a lot of harm. At least you'll know the money is real.