Yes yes, I realize Miley Cyrus is getting bad press lately so no one judge me but............ I used to love her style back in the later seasons of Hannah Montana and she had crazy long hair extensions. :)
For some reason (probably due to my insane love of makeovers:: see Pretty Woman, House Bunny, etc), I absolutely love this movie. Granted, there is not a LOT of information about the designers they used in the movie for Miley's character.
Here are the ones I actually found::
Kimchi & Blue from Urban Outfitters, Coach, Marc Jacobs, Vince Camuto, Rock & Republic, Armani Exchange
Then, I proceeded to research sorority dress attire/code in Lousiana (Baton Rouge area); which is where their costume designer, Wendy Chuck (from Twilight, btw!) went for research herself to find out what sorority girls wear down there. Then, I went from there.
There's a lot of J.Crew, obviously, sundresses, printed shorts, flats, cardigans etc....