A major emergency response was sparked when four pupils fainted during an Armistice Day service causing a "ripple effect" through the school.
Up to 40 children felt unwell as anxiety swept through Outwood Academy School in Ripon, North Yorkshire, a senior fire officer said.
One child was treated in hospital after banging his head, possibly after he fainted.
Fire crews from across Yorkshire turned out to carry out a safety sweep of the school, with a number of ambulances and police officers.
The specialists detected no sign of carbon monoxide poisoning or any other hazardous materials.
Station manager Dave Winspear said: "Children were feeling unwell, nauseous, dizzy. The ambulance service treated children for those symptoms.
"We worked through our protocol to see if there was anything untoward and assessed the school for any hazardous materials."
He said the emergency services did not know why the children initially felt ill, but said the hall where the 11am service was held was warm. "We are of the view the children just fainted and there was a ripple effect throughout the school.
"More children felt anxiety and started to feel concerned and the thing has escalated.
"I would like to assure you, we have carried out a very thorough assessment and found no hazardous materials or anything untoward in the school environment."
Pupil Joshua Hall, 15, said: "We were all in assembly for Remembrance Day and people were just passing out towards the end.
"It was pretty scary, it was talked about the whole day and it kept getting worse and worse.
"My dad didn't know what was happening, I didn't have my phone so I couldn't text him."
A parent, who did not wish to be named, 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."
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.
County Councillor Bernard Bateman praised the emergency services for their response and sympathised with the school's management who were constrained in what they could tell parents, once the emergency services took control.