Chugakusei Nikki

This is a love story between Akira Kuroiwa, a 15-year-old boy who knows nothing of love, and Hijiri Suenaga, 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: animenewsnetwork)
~~ Adapted from the manga series "Chūgakusei Nikki" (中学聖日記) by Kawakami Junko (かわかみじゅんこ).