I'm well aware that there are plenty of teachers working here illegally but I'd say that there are several reasons why I still feel that working on a z visa / residence permit are preferable.

You mentioned the need to leave the country and there is always the possibility of having to do 'the visa run' if you enter china on anything less thana z visa. This is time consuming and costly. Personally I've worked here for more than 4 years and have never had to make such a trip.

Recently you posted about your experiences regarding your dealings with officials when trying to renew your L visa including them trying to give you the wrong visa and then demanding to visit your accommodation. Again I can say that I've never had to experience such issues while working on the proper visa.

Finally, at least in my opinion, if an employer would bring you into the country illegally then I'd wonder what else they might expect of you. I know that a z visa doesn't guarantee you a good working environment but I think the lack of a working visa is something of a red flag when considering a position.

Of course not everyone is in a position to obtain a z visa and the risk is down to the individual but teachers should be aware of the potential difficulties of choosing to work on anything but a residence permit.