Hito ni Ienai Aidagara

Kensuke meets, through a dating site, a certain 'Michiru-san' who's erotic, has a sexy beauty mark near his mouth, and is great at playing coy. Although they both lie and claim they're 20, Kensuke is actually 17 and Michiru-san is 26. Not only are they 9 years apart in age, but Michiru also ends up being a teacher at Kensuke's high school!! As soon as the truth is out, Michiru stoically proposes to end the relationship.

(Source: Tokuma Shoten, translated)