Around 27 students have displayed signs of sickness and feeling faint at Outwood Academy School in Ripon, and at least two have been taken to hospital.
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It is not yet known what caused the students to fall ill and emergency services remain at the scene. The incident is believed to have happened between 11am and 11.30am.
Police said they were aware of two pupils who attended hospital or a local medical facility. The rest of the pupils were treated at the scene by ambulance staff.
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.”
A pupil, who also 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.
A police spokeswoman said: “Approximately 27 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.
“At this stage it is not known what has caused the students to fall ill and officers are working with the fire service to establish what has caused the incident.”
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.”
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.”