for Devilish by Maureen Johnson
The only thing that makes St. Teresa's Preparatory School for Girls bearable for Jane is her best friend, Ally. But when Ally changes into a whole different person literally overnight the fall of their senior year, Jane's suddenly alone-and very confused. Turns out, Ally has sold her soul in exchange for popularity-to a devil masquerading as a sophomore at St. Teresa's! Now it's up to Jane to put it all on the line to save her friend from this ponytail-wearing, cupcake-nibbling demon . . . without losing her own soul in the process.
Publisher description retrieved from Google Books.