Yuki-neechan no Kannou-gokko.

First-year high school student Shin Yuki stumbles one day upon his naked childhood friend, Yukino Kurose, reenacting a sensual scene with a mannequin in an empty classroom. She is a third-year student who wants to experience first-hand scenes from the book she reads and wants Yuki to help her! Things get a bit complicated though as her favorite books are erotic literature...

(Source: MU)