Tenshi na Konamaiki

Based on the hit Shonen Sunday manga and anime.

At the age of nine, Megumi is an aggressive boy prone to always fighting. One day he saves a strange man from a gang of other children. In return, Megumi receives a magical book. After accidentally bleeding on the book, a genie named Pierrot appears and offers to grant him a wish. Megumi wishes to become a strong man's man. Pierrot, a trickster, inadvertently turns Megumi into a woman. Megumi, furious, throws the book into the riverbank. Believing the only way to reverse the spell is to retrieve the book, Megumi begins a 6-year-long search but is told that she can find the book if she attends Furinkan High School.

[from Wikipedia]