Adachi Keiko manga
From A Duck's Paradise: Suri's a strong girl who stands up for herself and for the other girls! She's looked up by the students of her school, and she gets great grades. In short, with her two friends Eun-Bi and Ji-Sul, she's invincible! Except in front of her long-time crush, Jae-Sung, her smile turns into a creepy scowl... When she meets a good-looking boy with absolutely NO manners, she has enough dilemmas. But her brother, the Jjang of his grade, is in big trouble when a transfer student beats him... The only person the transfer student fears? His older brother--the boy Suri saw earlier!