Gal Bitch Shounen no Insei

Haru Awamichi, a student at an all boys' school, likes to cross-dress as a gyaru. Much to the gym teacher Inukai's surprise, none of the school's staff seem to mind, even though it goes against school regulations. After declaring to Haru that he will no longer allow this to continue, Inukai heads for the principal's office. When he arrives, however, he finds himself being drugged instead of getting the support he was seeking!

Upon waking up, Inukai finds himself sitting on a bed with his hands tied behind his back. He is confronted by Haru, who announces that the only way to make him stop cross-dressing is to participate in a very intimate "game." With no other option left, Inukai accepts, and the game begins!

(Source: MAL Rewrite)