Kim Ji-An, the Best Man

A loser named Kim Ji-an transferred to Sujung High School... But actually, he's the top dog of them all... and he's a girl!! Ji-an's parents, worried about their troublemaking daughter who's been in fights with boys in the past, want to send her only to an all-girls school. But Ji-an wants to attend a co-ed school.

In the end, she's allowed to go—on the condition that she'll disguise herself as a boy and keep it quiet... But it's already getting noisy!! -_-^

(Source: Naver, translated)