Kenka Bancho Otome -Girl Beats Boys-

Hinako Nakayama is a high school student who's grown up in state-run orphanages, unaware of her family background. She's approached by Hikaru, who claims to be her twin brother and reveals that they're the children of the Onigashima yakuza family's head. Hikaru asks Hinako to take his place at Shishiku Academy, an all-boys school notorious for its tough delinquents, so he can work on his own goals.