Sonyeojaepan

The unpredictable trial of a girl who was nothing at school begins!
Ordinary Park Ga-Eul witnesses the secret affair of Han Yu-Hyeon, who belongs to the so-called 'popular group', and uses this to blackmail Han Yu-Hyeon to join the group she admired...
(Source: Naver, translated)
Note: Chapter count includes 10 side story chapters