Seifuku no Koibito

From Translated Treasures: The story starts when an elementary school boy, Chiaki, brings his classmate & friend, Masamichi, home. There the irrepressible Masamichi meets Chiaki's older brother, Tomoya. Masamichi then proceeds to call him "cute," and ask him out. There's a five-year difference between their ages, so it's understandable when Tomoya freaks. The chase begins..and continues for years, while Masa works his way through elementary school, middle school, and high school.