Reluctant Hero by Melissa Mertz

The Secret Path of Ned the Ninja is a Middle Grade series about fifth-grade bully-magnet Ned Herts. Book One, Reluctant Hero, follows Ned as his uncoordinated feet wind up on karate mats thanks to a lousy day in gym class

Publisher Description

Hey… I’m Ned Herts. But you can call me Ned the Nerd. No, really… I don’t mind. My hobbies include: video games, comic books, creating superhero characters, bringing home awesome grades, tumbling into school lockers, and handing over my lunch money. Talents? Well, I have mastered the art of duck-and-cover while steering clear of Beck the Bonebreaker, my personal bully. Unfortunately, a super-bad school day involving a paper cut causes my parents to flip: They drag me to a karate class to toughen up.

Let me tell you something: When a guy like me lines up next to a cute girl to face his very first workout at the mercy of a weapons-happy martial arts teacher, it’s not pretty. But when a guy like me gets hauled into the woods in the middle of the night for another lesson, he’s going to wonder if superheroes can really exist and if his uncoordinated feet have just been placed on a very strange path.