Primary school children in Bacup, England were terrified during a school assembly when administrators told them World War III was happening and London was being bombed.

Staff at St. Mary’s Roman Catholic school led the children into a cellar when an air raid siren sounded, and set off a firework to stimulate the sound of an explosion, the Daily Mail reports.

Teachers planned the event as an exercise to educate the children about life during wartime scenarios.

“We were doing World War Two as a topic and we saw advice from schools that basically suggested that we introduced the topic as if it was really happening. We didn't foresee these problems,” said Mike Richards, headteacher of the school. “Unfortunately we made it too real. After that we spent all the rest of the afternoon explaining to them that it wasn't.”

Richards added that he had spoken to around ten parents who were concerned about the exercise, with the bulk of their complaints being that their children had nightmares after the simulation.

“You send your child off in the morning and then they are told that there is a war,” said one mother, who opted to remain unnamed. “Nobody with an ounce of common sense would have put them through it.”

Richards later apologized for the stress caused by the event.

Click here to read more on this World War III scenario gone wrong from the Daily Mail.