Spies Take Double Risk (Gallagher Girls Vol. 4)

In this novel, where the smartest girls on the planet go to a school to be trained as spies, Cammie Morgan's favorite teacher mysteriously disappears after trying to warn her of imminent danger. As she begins to investigate, she realizes that not everyone is who they say they are. And to top it all off, an ancient terrorist organization, the Circle, wants to kidnap her at all costs.