Tenshi Na Konamaiki

At nine, Megumi is already a brawler — always picking fights, always winning them. He rescues a strange man from a gang of children one afternoon and receives a magical book as thanks. A genie named Pierrot appears and offers a wish. Megumi asks to become a man among men. Pierrot, being a trickster, accidentally turns Megumi into a girl instead. Megumi throws the book into the river in fury. Six years pass. The spell is holding — unless the book can be found again. Somehow, it is at Furinkan High School.