@Sam *****
There is no problem coming in on a visa exempt or tourist visa entry, enrolling in a school that is approved by the MOE, having them issue the paperwork and you going to the immigration office where you live so you can get an in country 90 day Non-ED. (y) Totally doable
Most offices require a minimum of 15 days left on your current stamp to apply for an in country Non-Imm visa (Chiang Mai requires 21 days)
The reason that
@Raf *******
needed to get an extension before they apply for the visa was he didn't have enough time left on his current stamp.