Yo-Yo Girl Cop (2006)

Blurb:
Saki is a beautiful high school girl secretly working for the Japanese government as a "special student undercover cop." Her uniform includes a pair of see-through underwear. Attending a prestigious high school, Saki is sent to investigate a student prostitution ring in order to bail her estranged mother out of jail.

Once there, Saki gets bullied and emotionally abused by her classmates because of her underwear. Falling in love with a boy named Takuya, Saki is torn between love and duty.

Soon, she befriends a classmate/prostitute Asuka, who is later murdered when her double life is discovered. Joining forces with the girls from the school, Saki takes out her personal revenge on the prostitution ring run by the headmaster and a teacher named Reiko. Her weapon is a yo-yo and her battle cry is "I will never, ever forgive you!"

Source: www.anisearch.com/movie/4955