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.