Good Girls, by Laura Ruby (HarperTempest 2006).
Sixteen year old "good girl" Aubrey is humiliated when a photograph (by an anonymous photographer) of her and Luke DeSalvio, in flagrante delicto, is e-mailed around their school. Still, she manages to find the inner strength to carry on as she is shunned by family, friends, and classmates and starts hanging around with the school slut.
Good Girls is at once funny, poignant, and thought-provoking as it explores young love, relationships, and what it means to be a "good girl" or "slut." Highly recommended.
Cynthia interviews Laura here.