My eldest lived in hoodies and jeans at 6th form. It's good preparation for Uni. What kind of 6th form is your daughter going to that they have such draconian rules? If I was her I'd go as casual and as comfortable as possible within the rules and avoid anything that could be possibly construed as 'business casual'. Could be quite fun being as rebellious as possible.
So I'd suggest: cords, moleskin trousers, chinos, dungarees, mini skirts, maxi skirts, dresses, sweatshirts, jumpers, cardigans, football shirts, tshirts, long sleeve tshirts, sleeveless tshirts, casual shirts (DD1 has a vast collection from urban outfitters), DMs, knee high boots (or go to hell boots as my Dad called them), chelsea boots, loafers, Birkenstocks, mary janes, monkshoes etc etc.