I'm looking for Japanese manga

I looking for a bl manga , were there is top student that saw porn in his way home in the train on some else phone . He than got curious about how can man feels when the have something in their ass , anyway he started exploring himself and that led him to ignore his studies and his grades went down , the other characters was I think in the 2nd place ( he had crash on the top student) and he was shocked how the 1st top student score dropped and when he was trying to figure out the reason of the sudden score dropping he discovered that the 1st student enjoy anal play any that how they start interacting with each I think