Goodbye Lilac 

To high school teacher Kasai, Katou Reiji is a troublesome, yet cute student. Despite being confessed to by Katou, Kasai does not overstep student-teacher boundaries. Spring goes by and he sees Katou off at graduation. A few years have passed and Katou is now a teacher himself. And he happens to come to the school that Kasai works at... Note: Includes one extra chapter.