18+ Cool de M na Kaichou to Kageki na Amaicha Kozukuri Gakuen Seikatsu

The perfect chairwoman that everyone adores turns out to be a total masochist. He'll enjoy his sweety flirty and super stimulating academy sexlife with her! Anytime, anyplace, his M slave & lover will serve him, her master! She's begging for it!!

Nagisa is a childhood friend of his, slightly older than him.
She works as the academy as chairwoman and has had feelings for him a long time, but due to her overall excellence as a person, and a beautiful babe, she feels far away. However, he calls her on her birthday and there she gives him a confession. A confession of love. A confession that she is masochist. A confession that she wants to be his sex slave!

(Source: dlsite, edited)