The Gallagher Girls series by Ally Carter

"I suppose a lot of teenage girls feel invisible sometimes, like they just disappear. Well, that's me—Cammie the Chameleon. But I'm luckier than most because, at my school, that's considered cool. I go to a school for spies."

Cammie Morgan attends the Gallagher Academy for Exceptional Young Women. Although the school  appears to be an ordinary private school it is actually a girls' school for spies. Cammie is fluent in 14 languages and can kill a  man in seven different ways. She has been taught advanced martial arts and knows about the latest chemical warfare but she definitely doesn't know how to understand boys. 

The Gallagher Girls series was written by Ally Carter. It is a great combination of mystery and romance. It is written in first person as Cammie Morgan in informal language and it is a light-read. The series consists of: I'd Tell You I Love You, But Then I'd Have to Kill You, Cross My Heart and Hope to Spy, Don't Judge a Girl By Her Cover, Only the Good Spy Young, Out of Sight, Out of Time and the sixth and final book hasn't been published yet. I recommend the series for girls between 12 and 15 years old.