The key is in the door (1978)

Marie (Annie Girardot) teaches high school and has a 16-year-old daughter in her class. Divorced some years previously, she has some vague egalitarian notions about friendship with her students and leaves her door open to them. One of her protégés is found beaten up just outside her door, and an emergency physician comes by to treat her. When her daughter starts seeing someone she doesn't much like, and she begins having a brief affair (her first since the divorce) with the ER doctor, she begins to reconsider her policy.