Kubiwa Gakkyuu Boku wa Osananajimi-sama no Shimobe Desu

The protagonist, Yuuta, is a somewhat timid boy.
The story begins when he accidentally licks the recorder left behind by Hinagiku, the girl he has a crush on.

His actions are witnessed by Nogiku, Hinagiku’s mischievous younger sister and classmate, who even takes a photo of him in the act.
From that day on, he’s blackmailed into becoming her personal slave…

[Translated from dlsite]