Yuutousei Wa Mi No Hodo Shirazu summary:

Shougetsu, an honors student at the top of his grade, discovers the wonders of developing his backside. One day, a student from the same grade, Kyoujima, sees him anal masturbating in a classroom after school. In exchange for his silence, Kyoujima tells him to continue what he was doing...?!A curiosity-filled high-schoolers' pleasure study love!!