And off we go.
We mentioned it many times, get a visa that fits the purpose of your stay. The Visa Exempt and the tourist visa is not for long stay and to work to afford this stay.
The Non Immigrant ED visa is to sit in a classroom to study not to teach, but I understand, schools, directors, agencies and all those others does not make it easy.
For those in Chang Mai, times are changing at the moment, take care what ever you do:
Chiang Mai: Crackdown on tourists teaching at language institutes and cram schools
https://www.chiangmainews.co.th/page/archives/1174189