Naomi Uchiyama

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. (imdb)

Having spent time at a juvenile corrections facility, teenage delinquent Saki Asamiya is given a chance to redeem herself and delay her mother's possible death sentence by working as an undercover cop. Armed with nothing but a special yo-yo, Saki's assignment is to return to her high school and investigate on the corruption and mysterious deaths among the students. But in order to find the answers to the condition of her school, she must confront the wealthy and powerful Mizuchi sisters... (animenewsnetwork.com)