とら×うま ～本当にあった痛い話～

It is summer break, and Kotarou has devoted himself to jerking off in the mostly vacant school dormitory.
But then he feels someone's eyes on him. He turns to see that his door is slightly ajar.
Turns out that Yukie, the daughter of the dorm manager was still there!
"I think she saw me jerk off..."
It all began when he spent a week alone with Yukie, his childhood friend.

[From DLsite]