A legal teacher has a teacher's license (from the Thai Ministry of Education), a work permit (from the Thai Labor Department) and a one-year visa (from the Thai Immigration Dept)

The one year visa is issued on the strength of you having a work permit. If you don't get the work permit, you don't get the one year visa. They are inextricably linked.

An illegal teacher has no teacher's license and no work permit, and of course - no one year visa.

Illegal teachers are resigned to doing border hops and consulate runs to neighboring countries. Life is getting far more difficult in Thailand for this kind of teacher.

An ajarn reader writes - "I came into Thailand a week ago on a non-B visa with the intention of working at a school here.

I discovered on the first day that the terms of employment were not as they appeared to be. I informed the school that I would not continue with them and that I would need to look for other work.

Naturally they were not pleased and said they would go to immigration and cancel my non-B. Can they do that? Please help!"

Phil says "I'm not getting the full story here but I assume you fixed up a job while you were still in your home country and the employer sent you a letter of 'intent to employ' and you used that letter to get a non-immigrant B visa at your local Thai embassy.

The truth is that you haven't done anything wrong. It's not your fault that now you are actually here, the job has not turned out to be what you expected. 

It seems I can never repeat this enough but NEVER fix up a job before you arrive. Wait until you get here and then start looking for jobs.

Back to the original question - the employer can't waltz into immigration and start demanding that visas be cancelled. There's no contract between the two of you for starters. It's nothing but an idle threat. A pretty nasty one at that. I'm sure you've learned a lesson or two from all this though.  

Not sure why you would want to do that because you can legally work on a non 'O' visa as the rules currently stand.

It would be better to get the work permit on the Non-imm O visa then you don't have to rely on the teaching job to keep your legal status going.

You must have a salary of at least 40k baht per month, or show sufficient funds in a bank account in order to be approved and renew.

It's actually not a requirement to bring a criminal background check document (CBC) to Thailand, although some employers may ask to see one. 

The vast majority of employers have realised that generally teachers arrive in Thailand without the CBC and getting one from their own country or state is just too time-consuming and too much hassle.

The real problems can occur though when a teacher goes to a Thai embassy or consulate in a neighboring Asian country to apply for a non-immigrant B visa on the basis of obtaining work as a teacher. At some embassies and consulates, they will ask to see a CBC. Not all of them but some.

Getting yourself a CBC while you are in your own country is always a wise idea. There's every chance you won't need to show it but always be prepared for a first time.

The fact that you've been in prison for the crime committed (information withheld) won't matter.