Senior's Treatment - Mayumi Houjou in the Literature Club

"If you want me to keep it a secret, masturbate in front of me. Right here..."
The protagonist of the story is an ordinary Japanese boy who loves to write various sexual stories with his classmates in his diary. However, stories were devoted to cute girls from the literary club by the name of Mayumi. The guy described how he took her in various positions right in the library. And here's the thing, our hero lost his secret diary somewhere in the library... Soon, the hero was exposed Mayumi and now he will take a course of "treatment"...