A Social Studies teacher at Bell Middle School in Golden left a VHS copy of a Rosa Parks documentary for his class to watch while he was away at a meeting.
The substitute teacher filling in for him played the tape.
"His instructions were to show a portion of the tape, which he has shown for years, and then to stop it at a certain point," Lynn Setzer, spokesperson for JeffCo Schools said.
The students ended up seeing much more than they were supposed to. Eventually the documentary fizzled out and pornographic images appeared on the tape.
"The teacher had absolutely no idea that these images were on that tape. He is devastated that the children in this class were subjected to this and he deeply, deeply regrets it," Setzer said.
Parents received notification from a letter sent home with students.
According to the letter, "a substitute teacher was showing a movie about Rosa Parks ... [when] at the very end of the film, there was a brief picture of female nudity."
Bridget Jones, principal of Bell Middle School in Jefferson County, said the sub immediately turned off the film.
"[The teacher] and I are deeply apologetic for any discomfort this may have caused your child," wrote Jones in the letter.
Disciplinary measures have not been determined yet, according to Jones. However, the school is still investigating the personnel issue.
EDITOR'S NOTE: 9NEWS has decided to not name the teacher until the investigation has concluded.