Delinquent Detective (Sukeban Deka: Tanjô-hen) (1991)

There are some places a cop can never go, like high school. Blackmailed with the fate of her mother on death row, Saki Asamiya trades in her prison blues for a school-girl's uniform and discovers a pit of vipers festering in up-scale Takanoha High. The vicious Midzuchi sisters have the school in their grip, and they'll do anything to keep it that way. Saki, armed with a secret weapon that only looks like a toy, thinks differently.