Here are my suggestions regarding costumes:
1. Don't wear solid white (it will bleach out on camera) 2. No intricate patterns (vertical stripes can be especially deadly) 3. No brand names (We don't want Abercrombie and Fitch suing us) 4. No dangling jewlery, rustling fabric, or "noisy" clothing in general. 5. No strapless tube top things (weird if "head and shoulders" shots) 6. Hair should be out of the eyes/face. 7. Makeup - wear some. Sorry, that's all the advice I have :( Oh, and glitter makeup is/looks bad.
I know nothing about girl's fashion, I tend to say "wear something cute" and trust your judgement. In the real world, you might wear sweatshirt/pants when studying, but hey, we're in "Glamorous Movies" (or something like that.) It's always good to bring an alternate outfit, just in case the first one doesn't work out :)