Yes, Sir. Sorry, Sir!(2011)

Yes, Sir. Sorry, Sir! is a Hong Kong series that follows Law Yiu Wah, a former teacher forced to resign after a peeping Tom scandal. Now working as a police officer, Wah is tasked with returning to his old high school—a notoriously dangerous institution—to go undercover and investigate a triad drug operation operating within its walls. The assignment forces him to confront his past while navigating the complex world of organized crime infiltrating the school. Caught between his former identity as an educator and his current role as a cop, Wah must use his knowledge of the institution and its students to dismantle the criminal network before it destroys the school from within.