A US high school teacher could face disciplinary action after allowing her students to watch Fifty Shades of Grey in class.

The teacher, who claims not to have known about the sexual nature of the film, let her Hampshire County students choose a movie to watch as a reward for their hard work.

The screening was interrupted after ten minutes when the assistant principal walked past the classroom and recognised the film.

The principal, Jeff Woofter, said school policy prohibited students from watching any sexual movies in class.

“I will say it was an extreme lack of judgement from an otherwise very capable teacher,” Mr Woofter said.

“The teacher didn’t do a background check on the movie or even ask about it.”