Forty children at Outwood Academy School in Ripon, North Yorkshire, were treated for nausea and dizziness brought on by “anxiety”, prompting the mass arrival of paramedics, firefighters and police.
However, there was no sign of carbon monoxide poisoning or any other hazardous materials.
One child was treated in hospital after banging his head, possibly after he fainted during the service.
One pupil 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.”
More students complained of feeling sick and suffering from headaches later on but the emergency services gave the school the all-clear.
Station manager Dave Winspear, of North Yorkshire Fire and Rescue, said: “We are of the view the children just fainted and there was a ripple effect throughout the school.”