There are spies everywhere in China. I've seen and read about back when China started implementing the one child policy of how they would set up the Street Comission (don't know the offical term though in English) aka Nanny Patrol who would listen in on if a couple wanted a 2nd child. Of course there's always the cultural revolution which destroyed countless families who ratted each other out on trumped up charges of utter nonsense.

Foreign teachers NEED TO BE CAREFUL. What you even say or hint can have the local state security (guoan) up in arms and your computer may be hacked from the internet and you may be harrassed in real life as well. They're very good at doing it too from what I've read. It's hard to gather evidence against them if they choose to take action against you. Articles of what these people did to countless Chinese is mind boggling. Please be careful to all who decide to take up the journey to teach in China.