Shukujo no Tsuyagoto ~Hitozuma Miboujin no Nureta Kuchibiru, Karada no Uzuki~

The protagonist is a teacher at a prestigious school; Sae is a widow, she is the head teacher and is organizing the upcoming graduation ceremony, three other married women will help with the ceremony: Reiko who recently had a child and her husband does not make love to her ever since; Kaori who is a policewoman and its also sexually frustrated and Minori who was Sae's pupil and is the protagonist's friend.
One day the protagonist walking at night in the park witnesses Sae having sex with a man...will he use this opportunity to intimate with the hot head teacher....He also begins developing good relationships with the graduation committee, who consists only of sexually frustrated married women, who are more that happy to have a man that is interested in them...