LONDON, Nov 11 - Wednesday's incident occurred at Outwood Academy School in the town of Ripon, starting at an Armistice Day ceremony commemorating the end of World War I in a reportedly warm hall.

Dozens of other pupils then reported feeling unwell. However, emergency services called in to investigate said they could not find any trace of carbon monoxide poisoning or other dangerous materials.

"We are of the view the children just fainted and there was a ripple effect throughout the school," said Dave Winspear of North Yorkshire fire service.

"More children felt anxiety and started to feel concerned and the thing has escalated."

He added: "We have carried out a very thorough assessment and found no hazardous materials or anything untoward".

Police along with three ambulance crews, two rapid response vehicles, a doctor and other medical staff were sent to the scene.

One pupil at the school, 15-year-old Joshua Hall, told of how the fainting started.

"We were all in assembly for Remembrance Day and people were just passing out towards the end," he said.

"It was pretty scary - it was talked about the whole day and it kept getting worse and worse."

Another student, who did not want to be named, added: "When the first boy was sick, we thought it was a one-off but then there was a big slap on the floor and someone had fainted.

"After that it was a bit of a domino effect - another three or four collapsed and then people started leaving the hall to get fresh air ... they ushered us out quickly and a couple of girls had panic attacks."

The school, located near the picturesque Yorkshire Dales national park, has more than 600 pupils aged between 11 and 18.

AFP