Up to 40 school pupils simultaneously fell ill at a North Yorkshire school.
Students at Outwood Academy in Ripon showed signs of “sickness and feeling faint sometime between 11:00 and 11.30”, police said.
The ambulance service said children were treated at the school on Clotherholme Road. A hazardous materials team also attended.
The school confirmed four children collapsed during an Armistice Day service and 40 were treated on site.
A parent of one pupil, who wished to remain anonymous, said: “I rang the school to check my daughter was OK. I was told a couple of children had collapsed during a remembrance service.
“As it finished another couple went down. That’s when the fire brigade was called.”
Georgia, a pupil in Year 8, said people started being sick during an assembly.
Another pupil, who did not wish to be named, said students had collapsed in the school’s hall.
She said: “When the first boy was sick, we thought it was 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.
“One of the boys who fell was a bit concussed and had a big lump on his head.
“After that they ushered us out quickly and a couple of girls had panic attacks.”
The pupil said lessons went ahead as normal during the afternoon but more students complained of feeling sick and suffering from headaches.
A fire officer told the BBC they believed the hall “was a hot room and some children had become overwhelmed” and no chemicals or gases had been detected in the scare.
Fire station manager Dave Winspear said: “We are of the view the children just fainted and there was a ripple effect throughout the school.”
North Yorkshire Fire and Rescue Service said crews from Ripon and Boroughbridge were at the scene “alongside hazardous materials specialist officers”.
“A specialist detection, identification and monitoring (DIM) team from West Yorkshire Fire and Rescue Service is also at the incident,” the fire service added.
It is understood the school went into lockdown following the incident but pupils were later sent home.
They started to pour out of the school while police officers stood guard at the gates.
Parents gathered outside to meet the youngsters, with many of them hugging as they met.
Five ambulances remained parked outside as well as at least four fire engines with blue lights flashing.
In a statement, the school said out of the four pupils that fainted, one was taken to a walk-in centre for treatment for a “bumped head”.
It continued: “A total of 40 students were treated on site, however, all were able to be released back to their lessons, or at the end of the school day.”
Outwood Academy has 635 students, aged from 11 to 18.
On its website the school states it was judged “good” in its most recent Ofsted inspection in 2012.