Chiang Mai Thai Cookery School was very good. They have both standard and advanced courses. Even though I was a good cook and an okay Thai cook at the time, I took the standard course because I was with a novice friend and still had a very good time and learned a lot. (Ask questions!) We did a trip to a market in one and pounded curry pastes from scratch in the other. Every person got their own station with a wok and cutting board.
The Bai Pai classes look good as well, but I think Chiang Mai Cookery School is about $20/class whereas Bai Pai is $40 per class, with a discount for multiple classes. And you make about 7 courses at CMTCS as opposed to 4 at Baipai. (Also, there's more to do, imo, in Bangkok so it's better to fill your free time in Chiang Mai.)
I did CM Thai Cookery School too, and learned a lot and had a great time. The evening classes (more than twice as expensive as the day) allow you to get much more in depth, and that's where I learned the best stuff (there were 2 of us). I don't recommend doing both in one day, as it is way too tiring and too much food. The downside of this place is that day classes can get very big -- around 35 (if you are traveling alone, though, it's a great way to meet folks to hang with in the evening--I made some nice friends, and you are traveling in the off season so it should not be a problem). It's better to go for several days, and as said, ask questions and solicit feedback.
I also did a class at Sompet cookery school in Chiang Mai, which is much smaller, in a nice wooden setting above a river. It's less slick and professional, which was nice, but the teacher didn't quite have the English language skills that CM TCS does, which is pretty key if you are going to be asking a lot of questions, and I didn't take as much away from it.
Almost of all the places have copied each other's business model and menus, down to cooking almost exactly the same dishes in the same combinations (except the evening classes at CM TCS), so you won't find a lot of variation there.