Runaway (English, Paperback, Meg Cabot)

Emerson Watts is on the run: from school, from her family and from herself. She’s reeling from the shock that Nikki, the girl whose body her brain was forced to inhabit, is still alive. And she’s furious. Manipulative Brandon plans to use Em to discover a secret that will ensure his success, whilst Christopher is out for revenge.
Source- Goodreads.com