Yuuwakuppoi!

Main character is an ordinary student who has a crush on Mio. One day he has a lunch on the roof when gust of wind flips Mio's skirt revealing her secret part unprotected by underwear. Mio hurries to run away, but protagonist can't stop thinking about girl's reason for not wearing pants. When he comes to the room another day, Mio voluntarily reveals her secret part. That opens a door to new extraordinary secret school life.

[From vndbreview]