Gifted: Out of Sight, Out of Mind by Marilyn Kaye
Thirteen year old Queen-of-mean Amanda Beeson, wakes up one morning in the wrong body. She's become lonely, unfashionable Tracey Devon - one of Amanda's targets for bullying. Amanda discovers that Tracey, ignored at home and at school, has the ability to turn invisible. Amanda sets out to rescue her one-time victim from obscurity - as well as rescuing herself from Tracey's life!