Devilish — Maureen Johnson

Jane Jarvis and Allison Concord, seniors at Saint Teresa's Preparatory School for Girls, have always been too quirky to be popular, but at least they'll always have each other, right? Not so much. After a hideously embarrassing disaster, Allison comes to school transformed. Suddenly she has cute hair and clothes, speaks fluent Latin, and won't even look at Jane. With the help of a strangely wise freshman boy, Jane discovers the incredible truth:Allison has sold her soul to the devil. Now Jane will have to save Allison before the bizarrely exclusive Poodle Prom, a party of biblical proportions that just might blow apart the world as Jane knows it.