Many ESL teachers in Thailand are working illegally. For whatever reason they don’t have the documentation that would give them permission to work as a legal teacher. I would never judge these people, and realise that for some it is their only option. It does not mean that they are bad teachers in my opinion; although many of the less reliable teachers will be of this variety.
I previously worked as an teacher without the proper immigration documents. I was a volunteer teacher in my local school. I did this with the knowledge of the local police,village head, and a local man who worked for Thai immigration. They had no difficulty with me doing this unpaid work. The local school would not be able to afford the paperwork, and I wasn’t go to pay a lot of money to do volunteer work. Technically I could have been deported for doing this kind of work and not allowed to return to Thailand for eleven years. Apparently Thai immigration even view fixing your own car on the side of the road as work which you it would be illegal for foreigners to do.
I know that there are many more westerners who work as volunteer teachers without the proper paperwork. Some of these are retired and see at as a way to keep busy or put something back. Many of these retired guys probably wouldn’t be allowed to get a work permit on their current visa. They take the chance. Surprisingly, even the volunteer foreign police force which assists in tourist areas is full of people with no work-permits to do the task; the ultimate irony but this is Thailand.
Many illegal teachers are in paid work. They need to earn an income while staying under the radar of immigration. This is getting harder for them to do with immigration now asking more questions about how people are able to stay in Thailand with no obvious source of income. Visas are becoming increasingly more difficult to obtain. Illegal teachers do not usually get one year extensions so they need to do regular border runs or trips to Thai embassies in nearby countries; an expensive and time consuming hassle.
It seems that few of these western teachers get caught for working illegally, but they tend to only be able to get low paying jobs with little chance of advancement. Mind you, I’m in a low-paying job with little chance of advancement and I’m legal.