Osananajimi de Koibito no Kanojo to Ecchi na Shitagi

Aki asked his childhood friend girlfriend Koharu to wear a set of sexy lingerie he found in her room, but by the time they got a good mood going, it was time to go to school. So they went to school with her still wearing it under her uniform. She was clearly feeling it because of that — even in school; So he brought her to the infirmary...

(Source: Melonbooks, translated)