Hana-Kimi (3-in-1 Edition) Vol. 4-6 (manga)
It looks like Mizuki's cover might be blown when she finds out she's going to have to compete... in a cross-dressing match. She's going to have to be a boy pretending to be a girl - but that might give away that she's already a girl pretending to be a boy. Plus, a schoolmate has a crush on Mizuki - only he's super confused because he thinks she's a boy.