Story of a Girl by Sara Zarr

From Amazon: When Deanna's father catches her having sex in a car when she is 13, her life is drastically changed. Two years later, he still can't look her in the eye, and though Tommy is the only boy she's been with, she is branded the school slut. Her entire family watches her as though she is likely to sleep with anyone she sees, and Tommy still smirks at and torments her when she sees him. Her two best friends have recently begun dating, and Deanna feels like an intruder. She tries to maintain a close relationship with her older brother, but Darren and his girlfriend are struggling as teenage parents. Deanna learns to protect herself by becoming outwardly tough, but feels her isolation acutely. Her only outlet is her journal in which she writes the story of an anonymous girl who has the same experiences and feelings that she does.