GAKUEN HEAVEN [Manga]

Oshino Makoto just started as a temporary teacher at an all girls academy where he finds his childhood sweetheart working as a teacher.
Can his rekindled love survive the onslaught of sexy students begging Makoto to rescue them from the schemes of perverted teachers?

From Baka-Updates:
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. Unfortunately, he fails miserably and ends up as a substitute teacher in an all-girls private school governed by his grandmother. He meets a lot of female students there, especially Narusawa Tsugum; as well as a teacher who resembles his long lost first love, Asagiri Ayano. He even becomes the love interest of the substitute principal, his cousin Kyoko.