Shoujo-tachi no Saezuri

Shinya likes to masturbate in a school toilet. One day, when he sneaks into a school toilet for girls, he hears someone breathing hard. It's Kouko, his classmate. Also, he sees Rii kissing someone. After school, Rii suddenly says to Shinya, "If you keep what you saw secret, I won't tell anybody that you often sneak into a toilet for girls." Several days later, Satsuki asks Shinya about a relationship between Shinya and Rii. He soon finds out that the girl Rii kissed the other day was Satsuki...