Captain Underpants and the Invasion of the Incredibly Naughty Cafeteria Ladies From Outer Space (and the Subsequent Assault of The Equally Evil Lunchroom Zombie Nerds) by Dav Pilkey

Fourth graders George Beard and Harold Hutchins are a couple of class clowns. The only thing they enjoy more than playing practical jokes is creating their own comic books. And together they've created the greatest superhero in the history of their elementary school: Captain Underpants! His true identity is SO secret, even HE doesn't know who he is!