My Sisters Girlfriend (2016)

Pinku from 1981. Reiko Nemoto is a teacher. She had never had any experience with men until the age of 24, although this was not particularly intentional. Sanae Nishio, a schoolgirl, looks at Reiko as if she is an idiot. One day, while Reiko is inspecting the classroom, she hears the voices of a man and a woman. Looking into it, she discovers her colleague Kazuko Nishida having sex with Mamoru Araki, a student. Reiko is astonished. Kazuko, knowing that she was seen, invited Reiko to a coffee shop the next day…