Full Metal Panic!

Sagara Sousuke isn't your typical high school student. He reads military enthusiast magazines; he responds to questions with "affirmative;" he 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! (Source: J-Novel Club)

A sergeant by the name of Sousuke Sagara is assigned an important task. That of guarding high school girl Chidori Kaname. Sousuke is a military genius, gaining high rank at an early age, but because of his strange upbringing, he lacks social skill. Now, he finds himself as a fish out of water as he attempts to protect a girl from an evil organization and turn in his school work on time. (Source: Anime News Network)

Basically, it's a retanking of some chapters from Full Metal Panic!. It's sort of like FMP! Fumoffu.. meaning that there's no mecha plotline, only short stories. Also there's an extra story not presented anywhere in each volume of this series.