Yes, Sir. Sorry, Sir! Season 1 episode 2 (2011)

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YIU-WAH decides to apply for a police job and CHUN-BONG gives him tactical training but YIU-WAH still fails to get the job. However, he later receives a phone call from an employment agency and realizes that the Senior Superintendent of Intelligence Bureau of the Police Force, POON KWOK-SHING, wants him to be an undercover teacher in Lam Tin Kwong Memorial Secondary School for investigation of a campus drug case. YIU-WAH doesn’t really like to return to the school but he accepts the offer eventually for he really wants to be a real policeman. In order to collect information from the students, YIU-WAH changes his teaching style completely and he tries to get close to K4 in every ways but K4 simply ignores him. HO MIU-SUET, an enthusiastic new teacher, comes to teach Physical Education and General Studies in the school……