18+ Gakuen Tengoku

Gakuen Heaven is the story of Oshino Makoto in his attempt to become a teacher, in order to fulfill a promise made to his childhood friend and first love Koiwai Ayano, who also promised that when they see each other again they will get married. However he fails and ends up being a substitute teacher in an all-girls private school ruled by his grandmother. He meets there a lot of female students; especially Narusawa Tsugumi, as well as Asagiri Ayano, a teacher who resembles his long lost first love. He also becomes the love interest of the substitute principal, his cousin Kyouko.

(Source: ANN)