Double Cross (Mrs. Smith's Spy School for Girls #3)

Shannon Hale's Princess Academy meets Stu Gibbs's Spy School series in the third book in the Mrs. Smith's Spy School for Girls series.

Abby and her classmates have all been invited to Briar Academy to participate in The Challenge, a prep school competition where teams compete for prizes and the glory of being the best of the best. While there, they figure out their nemesis, The Ghost, is using Briar as headquarters to plan a devastating attack on his enemies (a.k.a.: pretty much everyone) using a brand-new invention Toby developed. And this time, The Center and Mrs. Smith will be of no help as Abby suspects there is someone working for The Ghost on the inside--and they can trust no one.