Kenka Banchou Otome

The heroine spent her entire childhood in state care with no knowledge of where she came from. Then a boy named Hikaru appears — her twin brother, separated from her at birth. Hikaru belongs to the Onigashima family, one of Japan's oldest yakuza clans, and his father has ordered him to enroll in Shishiku Academy — an all-boys school for delinquents, where the Onigashima heir is expected to establish dominance. Hikaru wants nothing to do with it. His solution: send his twin sister, whom no one there has ever seen, in his place. She agrees, disguises herself as a boy, and walks through the doors.