Chris and Angela, a couple living and working in Chiang Mai, have put together a fantastic overview of the Thailand multiple-entry tourist visa, which came into operation in 2015. All the info you need is there. 

What should officially happen when a teacher quits a job and hands the work permit back to the employer is that both teacher and employer should go to the immigration department and inform the officer that the teacher has terminated his / her employment.

The officer will then cancel the teacher's visa and the teacher has 24 hours to leave the country. However there are many 'ifs' and 'buts'. If the teacher needs to organise transportation out of the country, etc, - as is often the case - they can ask the immigration officer for a 7-day visa extension (at a cost)

In reality, when many teachers quit their jobs, their employer simply can't be bothered to go and do the right thing at the immigration office and the teacher ends up staying in Thailand for the remaining period that their visa allows. 

This is something of a risky game to play because if you, the teacher, are stopped by police for a passport spot-check, you no longer have a work permit to support the visa in your passport.

My advice - do the right thing. Get a one-week extension and then leave the country.   

You would probably have a work permit for the full-time job at the school, but the part-time work at the language institute would just be for a few hours in the evening.

One of Bangkok's biggest private language schools only employs part-time teachers who already have a work permit for their full-time job. This means that at the very least, the teacher is legally entitled to work in Thailand, albeit not for the part-time job.

However, an ajarn reader got in touch to say that at the back of the work permit book, there is space to add a 'second employer'. So it seems you can possibly cover two jobs at two different locations with just one work permit book.

Firstly, this is by and large the employer's responsibilty and not the teacher's.

You need to liaise with your school's admin person and tell them that you are leaving (hopefully you gave them 30 days notice) and discuss this issue.

Ask the employer to inform you as soon as your work permit has been cancelled and returned to the labor department. You must ensure that this procedure has been carried out or else getting your new work permit for the next job could be a problem.

Years ago, it was the responsibilty of the employer to give the teacher a chit to show that the permit had been cancelled but that's no longer the case. Therefore the current system is open to all sorts of administrative procrastination if your school is that way inclined.  

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.   

Firstly, there is no such thing as a work visa in Thailand. You mean a non-immigrant B visa, which allows you to apply for a teacher's licence, etc.

Many people take the option of returning to their home country because non-immigrant B visas can be more difficult to obtain in the Asian countries bordering Thailand, especially if you don't have the correct paperwork.

No. It's the responsibility of the teacher to provide criminal background checks to the employer (if required)

Most school admin departments wouldn't have the first clue how to go about the process anyway,

Firstly, there isn't a lot of demand in Thailand for some kind of substitute teacher who flits between different schools perhaps covering for absent teachers, etc. In truth, there never has been a demand for this sort of position.

Maybe one or two of the larger private language schools who supply teachers to Thai secondary schools might sometimes have a vacancy for a supply teacher, but you would still be classed as a 'full-time' teacher for that particular private language school (agent)

The thing is you still need a work permit to do this type of work so technically you need a non-immigrant B visa in order to start the work permit process.

Visas don't entitle you to work legally (there is no such thing as a 'work visa' in Thailand) To work legally, you still need a work permit. And it would be nigh on impossible to get a work permit as a substitute teacher working for several different employers. 

You should ideally bring your original degree certificate and originals of any transcripts. And also a criminal background check if you can get one (although these seem to be less and less of a requirement as time goes by)

Don't fret about how many copies of each document you need to bring. It's not as though Thailand doesn't have photocopiers (as some people seem to think!)

Yes, this can be done if you are a teacher with a guaranteed job offer but surprise! surprise! there is a certain amount of hassle involved.

A lot of the responsibility rests with your employer, who will need your paperwork in order to apply for a letter from the Thai Ministry of Education. This letter can take anything from 3-6 weeks to obtain.

Because of the hassle involved, many employers take the easy option of asking the teacher to go to a neighboring country such as Laos or Malaysia and apply for a new non-immigrant B visa from a Thai consulate or embassy in that country.