Bee-be-beat it! manga

Bee-Be-Beat It! summary:

Dokuritsu Tenjin Gakuen is a special school that give grades through "Battles" instead of tests. So everyday, the students engage in all-out battle and get points based on their rankings. Two students Hattori Kyousuke and Kujou Hachie (aka Hachi) are childhood friends. While Hachi is the strongest freshmen girl of the school, Kyousuke is just an above-average student, but he cannot remain mediocre forever because Kyousuke made a promise: to defeat Hachi and marry her, or else Hachi is forced to wed her perverted cousin. Can Kyousuke get strong enough to fulfill their promise?