Sukeban Deka Pretend

Saki Asamiya is a fearsome high school delinquent imprisoned in an inescapable juvenile detention center. Her chance at freedom comes when she accepts a deal from the police: infiltrate Japan's elite schools to investigate crimes the cops can't touch, and her mother will be spared from execution. Undercover as a regular student, Saki sets off to unmask criminal conspiracies, wielding a custom steel yo-yo as she faces off against corrupt teachers and scheming students. (Source: Omoi)