''It was indeed very stupid for me to give original documents to my employer,....''

In the UK you wouldn't be able to obtain a full-time public school teaching post without submitting originals to your employer in the first place. Of course, they would be returned to you intact timeously by registered mail after having been checked. I suspect that that would also be the case in other developed countries in the West. So you needn't blame yourself too much for what has happened to you here in the PRC. You weren't to know in advance.
You'll surely have heard of the Wild West; well,this is the Wild East in the eyes of many foreigners. What has already happened to you is par for the course here. Extra precautions are definitely required. Cast aside your Western assumptions,they won't always hold true here.