Jugyo Sankan no Onna (1984)

Original title: 授業参観の女

One spring, Tasaka, an elementary school teacher, receives a new class of first graders. While taking the children's attendance, Tasaka is surprised to see a call girl he once had an encounter with standing among the parents in the back of the classroom. According to the school's files, her name is Yukako Sawada, and her child Eisaku has no legal father. The next day, Eisaku gives Tasaka a letter from Yukako asking to meet at the hotel where they first met, and Tasaka sets out with an uneasy feeling...