Working Visa
Type of Visa – The type of visa you should apply for at your nearest Thai Consulate or Thai Embassy is the one year multiple entry non-immigrant B visa. This will enable you to enter the Kingdom of Thailand for 90 days at a time, and is currently the best visa for foreign language teachers. If you utilize your visa the last time just before it expires, you will get an extra 3 months "bonus", so the total validity will be nearly 15 months.
It is best to apply for a multiple entry visa in your home country rather than Thailand's neighboring countries. Thai consulates in the United States, Europe and Australia seem to be the most generous, but Thai Embassies issue the visa as well.
Documents for Visa - To get a non-immigrant B visa you need documents from your placement school to present to a Thai Consulate/ Embassy in another country outside of the Kingdom of Thailand.
One of these documents is a letter stating that the applicant has been offered a job and that the school requests that the applicant be given a non-immigrant b visa. They also ask for registration documents from the company you are going to work for. These documents will be given to you by your placement school once you have confirmed your placement.
Recently Thai Consulates have been requesting a police check before they will issue the visa.
You must realise that acquiring this visa is your responsibility, and a phonecall to your nearest Thai Consulate to check details of exactly what you need to do is sensible.