BBC said:Up to 40 school pupils have simultaneously fallen ill at Outwood Academy in Ripon.
They showed signs of "sickness and feeling faint sometime between 11:00 and 11.30", police said.
Yorkshire Ambulance Service said children were being treated at the school on Clotherholme Road. A hazardous materials team also attended.
A parent of one pupil said children collapsed during a Remembrance Day service to mark Armistice Day.
The man, 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 were 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 several 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.
BBC said:Some have described the mystery of the collapsing students as "mass hysteria"