She's on Duty (2005) on Netflix

Plot summary

An orphan who became a police detective, Chun Jae-in is handed an assignment she does not want: enter a high school as a student and get close to Seung-hee, the daughter of the gangster Cha Young-jae. The task has two sides: keep Seung-hee out of the reach of her father's enemies, who might take her as bait, and learn what she can about his work. Her uncle, also a detective, presses her to accept at the last minute. On her first day her classmates take her for a loser, and a group of gang girls challenges her to a fight in the schoolyard, which she wins easily. Kang No-young, a classmate who lives next door, helps her toward Seung-hee's friendship.