Shousetsuka Sensei no Nagakatta Koi

Mishima, who works for a housekeeper dispatch agency, is sent to the apartment of a popular novelist, Matsudo Sao. When Mishima visits Matsudo's house on his first day of work, he is met by Aso, his homeroom teacher from junior high school...! Mishima, intrigued by the insecure Aso who's acting suspicious, even though he is now a popular author, reads his debut novel and notices a passage.
The main character, a female teacher, falls in love with her male student, who was the very same Mishima when he was in junior high school.
At the time, Aso was in love with Mishima, his student. And ever since then...

(Source: Homesha, translated)

Note: Includes one extra chapter.