Hey ladies! I have been living and teaching at a private school near Bangkok for about two months and here are my observations on the female teacher dress code.
When I first arrived, I was very surprised to find out that, not unlike 1950s Americana, dresses and skirts are the expected, encouraged, and unofficially required of all female staff and faculty at the school. So, before your first teaching gig, don't waste your money buying pants. Maybe the school you work at will not be as strict with this policy, but unless you have overwhelming feelings about this topic, buy skirts and dresses to be on the safe side.
On that note, it seems that unlike in the U.S., where this is sometimes stigmatized, wearing short skirts with hairy legs is acceptable. I have seen many high school and university students sporting this look, and the occasional teacher too. I have never seen anyone wear pantyhose here; I am very thankful for this, it's far too hot out there for such things.
Necklines on shirts should remain high. Try to avoid showing any clavicle, and cleavage is out of the question. Iron your shirts, or pay someone to iron them.
Like with men, personal body odor is highly offensive here. This may pose some problems for farangs from cold environments, because you'll sweat a lot when you arrive here -- I know I did! This means you'll have to wash your clothes more frequently. The good news is that the clothing here is less expensive than it is in the U.S., so when it wears out from washing, it's somewhat easier to replace (if you can find Thai clothing that fits). Most women here don't wear socks with dress shoes, but you might want to think twice about this fashion move if you're prone to foot stink. At my school, if a teacher smells of cigarettes, etc., it is a BIG deal. This is an understatement.
It's not a bad idea to look polished as far as hair and make-up are concerned. I have noticed that many of the female teachers here are experts in that department, Still, my hair is usually free-form, and my make-up varies depending on how much time I have to get ready for work. So far, nobody has complained.
Important note: Yellow and red are colors for two rival political groups. Be mindful of your environment when wearing these colors.
I hope this was helpful. I hope some of you female farangs reply with some of your observations and reflections too.