Full Metal Panic? Fumoffu (2003)

It's back-to-school mayhem with Kaname Chidori and her war-freak classmate Sousuke Sagara as they encounter more misadventures in and out of Jindai High School. But when Kaname gets into some serious trouble, Sousuke takes the guise of Bonta-kun - the gun-wielding, butt-kicking mascot. And while he struggles to continue living as a normal teenager, Sousuke also has to deal with protecting his superior officer Teletha Testarossa, who has decided to take a vacation from Mithril and spend a couple of weeks as his and Kaname's classmate.