Oshiego to Kyôshi: Itazura Hime Hajime(2003)

Tatsuji, the protagonist, is a retired high school teacher living a quiet life of leisure. With time on his hands, his only pleasure is eavesdropping on his son and daughter-in-law’s sex life. One day, a former student named Kana visits him and declares, "Teach, have sex with me!" To think he’d be giving a girl like her a "private lesson" in bed...