Full Metal Panic!

Jindai High School was a relatively safe place, right up until Sousuke Sagara transferred there. A total nut, he wreaks havoc on the school's student body and gets into a world of trouble with the teachers! But there's more to Sousuke than meets the eye—not just a hell-raising student, he's actually a member of an elite military unit, on an undercover mission to protect the beautiful schoolgirl Kaname Chidori from the KGB.

(Source: TokyoPop)