The man, 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."
Police said the incident is believed to have happened between 11am and 11.30am.
A spokesman for North Yorkshire Fire and Rescue Service said specialist hazardous materials officers were at the school.
"We are working with colleagues from North Yorkshire Police and Yorkshire Ambulance Service at this time to determine what has caused this occurrence," he said.
- Press Association
- AAP