The kids had settled down to a maths lesson at The Regis School in Bognor Regis, West Sussex, and were using an interactive white board to watch a YouTube video.
But one of the pupils selected a trailer from the sick 18-rated horror film The Human Centipede – and the teacher allowed them to watch it.
They were left horrified as they saw a mad scientist hunting his victims, ending with gruesome shots of three people with their body parts sewn together.
Parents reacted with fury when they found out.
Mum Fearn Cranston, 38,whose son Jack, 12, was in the Year Seven class, said: "My son told me it was disturbing and that he was scared and frightened so I looked it up, and when I saw it I couldn't believe what I was seeing.
"I'm still disturbed by it now, it was absolutely disgusting and almost bordering on porn.
"You can tell it is going to be a horror as soon as the trailer starts playing. The teacher should have then intervened but he let the kids watch the whole thing.
"I can't believe he didn't stop it, you don't want your 12-year-old child watching things like that at all."
A school spokesman said: "It is unfortunate that a class of students were shown a short trailer for the film.
"This was an error of judgement and the matter is being dealt with internally."