The Year of Secret Assignments by Jaclyn Moriarty

Cassie, Lydia, and Emily, students at Ashbury academy have been assigned a pen pal project with Brookfield, a rival school, to learn the "Joy of the Envelope." The girls write to, and receive letters from their Brookfield pals, Matthew, Sebastian, and Charlie. Soon there are secret missions, mistaken identities, romances, and an all-out war between the schools errupts

If you're looking for a great, funny, quick read- this is the book for you. The book is full of humor, mystery, and great hijinxs. The characters are wonderfully developed, the story is entertaining and fun