Chuugaku Seinikki

Akira Kuroiwa is a 14-year-old boy who knows nothing of love, and Hijiri Suenaga is his new 25-year-old homeroom teacher. One fated summer, Akira begins to realize he has feelings for Hijiri, but Hijiri has a long distance fiancee. The story follows the pair over the course of several years.

(Source: MangaUpdates)