Fight Back to School 2 (1992)

Star Chow is an officer in the Royal Hong Kong Police’s elite Special Duties Unit (SDU). During a meeting with his senior officer, Inspector Yip, Star jokingly suggests he wishes to be reassigned to the traffic unit.
Star immediately finds himself demoted to Constable engaging in traffic duty on the streets of Hong Kong. After being made the scapegoat for a failed high school terrorist investigation, Star hastily resigns from the police.He decides to enroll at the high school to launch his own private investigation. But Star realises the investigation won’t be easy when he discovers that the bumbling, incompetent CID detective Tat is also undercover at the high school.