I dress quite smartly for school. I'm Head of VI Form and head of a dept, and feel that my clothes should reflect that. Also, I've just gone back full time after 4 years wearing sloppy jeans, t-shirts, trainers and baby vomit, so it's quite a treat to wear smart trousers, dresses, jeans and heels. It makes me feel like I'm "at work".
Quite a lot of the middle management dress smartly. There's no dress code, as far as I'm aware, but we have a real mix. On the whole, TAs are less formal, but I've never seen anyone in jeans. Leggings seem quite common though.
I've taught in two other schools. In the first, the "young" staff were spoken to at a special staff meeting about dressing more smartly than we did - that put a few noses out of joint. It was my first job, and I hardly had the cash for new clothes as it was. At the other school, dress was very informal - several (including me on occasion) wore jeans and no one batted an eyelid.
TBH, I've always assumed schools will leave it to staff to use their own judgement about what to wear - anything else seems a bit patronising!
BTW, is the no flipflops thing being touted as Health & Safety?