Yuki-neechan no Kannou-gokko.

First year Highschool boy Shin Yuki stumbles one day upon his naked childhood friend reenacting a sensual novel with an mannequin in an empty classroom. Yukino Kurose, a third year student, loved to reeanact stories with Yuki's help from books when she was a child, to discover how the characters in the story felt.
This time she got herself a sensual novel and asks Yuki to help her out with her reenactment again. Otherwise she'll have to find someone else, because mannequins are not enough.
After a quick deliberation Yuki decided to help her out, because he doesn't want anyone else to see her naked