Jokyōshi okasu (1998)

Rinko, a high school teacher, is the object of admiration for her male students due to her looks and intelligence, and her single colleague, Nozawa, is also infatuated with her. One of her students, Yuichi, also harbors a secret crush on Rinko. One day, Rinko comes to Yuichi's rescue when he is being bullied by other students. However, Yuichi feels humiliated that he was helped by the woman he admires, and he stops attending school the next day. Worried, Rinko calls Yuichi's house, and his older sister, Etsuko, answers the phone. Etsuko showers Yuichi with affection that goes beyond that of siblings, and even helps him with his sexual needs. Etsuko asks Rinko to come visit him. What awaits Rinko there is...