Pretty Little Devils by Nancy Holder
from the publisher's site:
Hazel Stone wants nothing more than to be a part of the hottest clique in school, the Pretty Little Devils, but she’s stuck at a lunch table full of high school C-listers. Hazel has resigned herself to life as a nobody—when suddenly everything changes. The PLDs invite Hazel to one of the group’s famous parties, held at the site of one of their babysitting jobs. Before Hazel knows it, she’s in with the in crowd—and she couldn’t be more thrilled!
But nothing turns out the way she expects. Especially when one her classmates becomes jealous of her newfound status—deadly jealous.
Author Nancy Holder weaves a wicked tale about the price of popularity, and having the kind of friends some girls would just die for.