From J-Novel:
Sagara Sousuke isn't your typical high school student. He reads military enthusiast magazines, responds to questions with "affirmative" and brings grenades to school in his bag. Though everyone at school takes him for a hopeless military geek, Chidori Kaname thinks there might be something more to him. When their plane is hijacked in the middle of a field trip, Kaname's instincts will prove correct: Sousuke is an elite, mech-piloting mercenary and he's here to protect her.