Yuki Nee-chan no Kannou Gokko.

One day, Shin Yuki, a first-year high school student, discovers Yukino Kurose, a friend from his childhood who is naked, acting out a sensuous scene with a mannequin in an empty classroom. She is a third-year student who wants Yuki to assist her in experiencing scenes from the book she reads firsthand! However, because her favorite novels are erotic fiction, things become a little more tricky.