Do I need a student visa to study Thai massage in Chiang Mai for 3 months?

Graeme, I recommend you join the Chiang Mai Massage Students page. You will find more rational response to your interest beyond visas. Those who said you can study on any visa were correct. As a licensed massage therapist in USA I did that in 2014 on visa exempt. For a longer stay the tourist visa and an extension would suffice for 90 days. Thai massage is powerful because it combines direct deep tissue work and Asian medicine focus on the Sen lines, aka meridians. I studied with Jack Chaiya, but there are many options.

No need to go to a school just go to a massage shop and ask them to train you

Legally you won’t be able to change schools. It’s your school that supports your ED visa so you can’t go hopping around like that. Whether your school will actually give a shit is a different question. But legally if they catch you not being educated at the massage school you have on file…yeah good luck

A person can study any subject on any visa. I recommend getting a single entry tourist visa, and extending it once by 30 days. It will give you a 90 day stay.

I hope this has a happy ending for you.