There is a ton of massage schools in Chiang Mai, and some of them are pretty questionable. The big ones, Old Medicine Hospital and ITM, are very official and great if you want to get a very structured education. I had a great experience at a school around the corner from ITM called The School of Massage for Health. The instruction is fun and professional, and you learn at your own speed. It is primarily one-on-one instruction, and one is able to practice on the instructor to fine-tune technique. The director, Picnic, often takes all the students on random but wonderful field trips: hot springs, other massage schools, herbal pack massage places... and at no extra cost. Their courses are flexible and accommodate the amount of time you have. This school rocks!
Info: 10/1 Chareonsuk Road (near Santitam)
Phone: 053404028, 096924445, 098525231