More than two dozen students fell ill at a secondary school after children began collapsing during a Remembrance Day service.
Around 27 pupils showed signs of sickness and feeling faint at Outwood Academy School in Ripon, North Yorkshire Police said.
Two children are known to have been taken to hospital or a local medical facility, while the rest were treated at the scene by ambulance staff.
It is not yet known what caused the students to fall ill and emergency services remain at the school.
A parent of one pupil said two children collapsed during a ceremony to mark Armistice Day.
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 are at the school.
"North Yorkshire Fire and Rescue Service received a call at 12.21pm to reports of a number of pupils feeling unwell at the Outwood Academy in Ripon," the spokesman added.
"Crews from Ripon and Boroughbridge are at the scene alongside hazardous materials specialist officers.
"A specialist detection, identification and monitoring team from West Yorkshire Fire and Rescue Service is also at the incident.
"A number of students have been affected who have displayed signs of sickness and feeling faint. Some have been attended to by paramedics at the scene and others have been taken to hospital.
"We are working with colleagues from North Yorkshire Police and Yorkshire Ambulance Service at this time to determine what has caused this occurrence."