Jake and the Dynamo by D.G.D. Davidson
Jake Blatowski can’t wait for high school: basketball, calculus, and a cafeteria that isn’t under investigation by the health department. Well, he’s going to have to wait: a computer malfunction has assigned him to the fifth grade.
It’s bad enough that he bangs his knees on the desks or that Miss Percy is going over long division … again … but Jake’s sitting next to Dana Volt. She’s a perpetually surly troublemaker who doesn’t even have to exert herself to make his life a living hell. But no, it gets better: Dana secretly belongs to a coalition of girls protecting humanity from the horde of deadly monsters that plagues the city. But Jake’s no hero; he just wants to get to varsity tryouts!
When the monsters choose a new target, Jake’s not at all surprised that the target is him. Sure, why not? That’s the kind of week he’s having. Now the impulsive and moody Dana is the only one who can save Jake from certain death—but Jake is the only one who can save Dana from herself.