The Year of Secret Assignments — Jaclyn Moriarty

Three girls. Three boys.
Two rival schools.
This could be messy.

The Ashbury-Brookfield pen pal program is designed to bring together the two rival schools in a spirit of harmony and "the Joy of the Envelope." But when Cassie, Lydia, and Emily send their first letters to Matthew, Charlie, and Sebastian, things don't go quite as planned. What starts out as a simple letter exchange soon leads to secret missions, false alarms, lock picking, mistaken identities, and an all-out war between the schools -- not to mention some really excellent kissing.