If you don't have a work permit this is illegal. As noted on your passport it is illegal to work in Thailand. Teaching English and drawing a salary from Thai English school is just that. Make sure that you have a work permit before starting the job. Many of these schools take advantage of the naive and anxious foreigner stating that they will obtain it later.
Many English schools deal with teachers like cattle.
They know someone else will be right behind you to take your place. This is a case of ship them in ship them out.