Nobara No Mori No Otometachi

From here:
Hatsumi and her best friend Sakura have just entered the prestigious girl’s academy, Otowa. The two girls promise that no matter who they meet, they’ll always like each other best. This is fine and dandy until Hatsumi meets Izumi, the beauty of the academy who likes to dress like a boy. Izumi keeps turning up to help Hatsumi, and Hatsumi starts to develop a crush on her… until she sees Izumi kissing another girl.