Innocent Thing (2014) on Netflix

A high school student develops a crush on her gym teacher.

Former physical athlete and father-to-be Joon-Ki works as a PE teacher at an all-girls high school, where many of the students find him to be an attractive man. Young-Eun is a particular outgoing and brash young girl whom is very straight forward and is creative in finding ways to interact with Joon-Ki, whom is the center of her crush. During one fateful rainy day, Joon-Ki is alone in the classroom with Young-Eun and things go too far. Joon-Ki tries very hard to correct the errors of his ways and wants to put it behind him, but the smitten Young-Eun's obsession with him seems to know no limit. And soon Joon-Ki finds himself with a shattered marriage.

—Paul Magne Haakonsen