She's On Duty (2005) (KOREA)

She's on Duty (잠복근무 Jambok-geunmu) is a 2005 South Korean film about a police detective who goes undercover in a high school to befriend the daughter of an elusive gangster. Despite being heavily marketed the film was generally considered a box office disappointment. It stars Kim Seon-ah as Chun Jae-in, Nam Sang-mi as Cha Seung-hee, Gong Yoo as Kang No-young, Kim Kap-su as Cha Young-jae, and Kim Sang-ho as Detective Kang.

See:
# She's On Duty at Wikipedia