Hi ben,
As I've only ever been to Jomtien/Pattaya, the only school I know of is
the Amara Sailing Center.
Here's a link:
http://windsurfing-thailand.com/index.html
I'm sure there are others, possibly closer to Bangkok/Phuket but I'm sure that Ms. Amara can direct your to something closer if you need it.
Hope this helps,