Around 40 students have been treated at a secondary school after children began collapsing during a Remembrance Day service.
The 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.
Yorkshire Ambulance Service said its crews treated 40 at the scene for "nausea and feeling faint".
"We dispatched a number of resources to the scene including a doctor, three ambulance crews, two rapid response vehicles, a number of Clinical Supervisors and our Hazardous Area Response Team," a spokesman said.
"Up to 40 students are currently being treated on scene, but we have not conveyed any patients to hospital at this stage."
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."
It is understood the school went into lockdown following the incident but pupils are now being dismissed.
Julian Smith MP, who represents the constituency of Skipton and Ripon, said: "I am in touch with the school and I'm assured that the emergency services are responding and the situation is under control.
"The Secretary of State for Education has been in touch to offer any support required from her department and I will be assisting in any way I can to help the school, parents and pupils."
Children have now started to pour out of the school as they leave to go home, with police officers standing at the gates.
Parents of the children gathered outside to meet the youngsters with many of them hugging as they met.
Five ambulances remain parked outside as well as at least four fire engines with their blue lights flashing.