No ties. Seriously, it’s just too hot.
Long pants for the guys, modest skirts, dresses or pants are okay for women.
In the past it’s been considered a bit immodest to show the shoulders or stomach of a woman in public. This is a general statement, but if you tour temples you will not be allowed in with shorts, bathing suits, bare shoulders or midriffs. Sort of like going to church here, with a lot less leniency.
Shoes are not that important, as Thais have a sort of reverse foot fetish.
Take off your shoes if inside a home or any room where someone might sit down, but not in shops or other places that have high traffic, unless others are doing that.
When you do sit, try not to cross legs or leave your legs sticking straight out. Sit with your feet beneath and behind you if possible (semi kneeling). “Indian style” is okay, just not crossed at the knees.
Take inexpensive shoes to wear to temples. You can’t wear shoes in a temple and there’s a slight but real chance that nice shoes will be stolen while you’re inside of one.
Loose clothes, light natural fiber clothes will be more comfortable, but are probably not stylish.
I’m not going to go too much into style for a couple reasons. Primarily I’m probably off by a few years, but also unless the students are Thai or Chinese they’ll tend to stick out anyway, so no real reason to adhere to Thai fashion. This also is somewhat true with some of the rest of the above. Some stuff will be forgiven as you’re anyway “farang” and don’t know any better. That said many younger Thais tend towards jeans, usually cuffed up, sometimes to a sort of “capri pants effect”, with a t-shirt, shirt with collar, or blouse.
Most of the stuff in guide books is a little dated or over hyped. It’s probably best to avoid trying to “wai” people and just smile and/or nod if someone wais you, as you won’t know the rules and they can get complicated. As a general rule, you should probably only actually return a wai to someone with whom you would shake hands in day to day life. So for example returning a “wai” to a doorman or server is a little silly, but people will do it. Also the person of inferior position should wai first, which is a bit on social status, family heirarchy and age. If in doubt don’t.
BTW, December is the best time to travel there and peak season. You should have a blast. Maybe bring some gifts for whomever you visit or meet. These should be from the USA (or whatever country you’re coming from) but need not be expensive.