She's on Duty (2005)

Detective Chun Jae-In (Kim Sun-A) goes undercover as high school student to befriend Seung-Hee Nam Sang-Mi, the teenage daughter of a powerful crime boss, but, Chun Jae-In doesn't remember high school being this hard. Not only does Seung-Hee give Jae-In the cold shoulder, but Jae-In finds the math problems incredibly difficult and she has to deal with the school bullies. Meanwhile, handsome student No-Young (Gong Yoo) enters the picture and Jae-In finds herself falling for him!