Cross My Heart and Hope to Spy by Ally Carter

I highly recommend Cross My Heart and Hope to Spy. It's the second book in the series, the first book being I'd Tell You I Love You, but Then I'd Have to Kill You. The main character, Cammie, thinks maybe things can settle down at her secret school for spies after the breakup with Josh, who put the school's security at risk, but why would it do that? Then, a group of boys from the code name Blackthorn comes to her school for the first time. She finds herself trying to figure out Zach and the truth about the new school. Along the way, multiple obstacles occur, and she starts wondering if she can do it. Anyone who likes a nail-biting but romantic story won't want to put it down.