A pupil at the school in Ripon where up to 40 pupils have been treated for sickness said the students began to feel unwell during the minute's silence taking place to commemorate Armistice Day.
Olivia Craft told ITV News pupils started to collapse just after 11am.
Pupils are being treated at the school, but none have been taken to hospital at this stage.
Yorkshire Ambulance Service has issued a statement saying up to 40 students are currently being treated on the scene at Outwood Academy, and not 27 as originally reported - but they have not taken any to hospital at this stage.
Yorkshire Ambulance Service received an emergency call from Outwood Academy School in Ripon at 11.30am this morning to reports that a number of students had fallen ill suddenly with 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. Forty students are currently being treated on scene, but we have not conveyed any patients to hospital at this stage. The cause of the incident is unknown and emergency service colleagues from North Yorkshire Police and North Yorkshire Fire and Rescue Service are at the school carrying out investigations.
The MP for Skipton and Ripon has tweeted offering his support for Outwood School, where 27 pupils have been affected in a health scare. He says Education Secretary Nicky Morgan has also offered her department's help.
North Yorkshire Fire and Rescue Service say its Hazardous Materials Specialist Officers are investigating the incident at Outwood Academy, along with crews from Ripon and Boroughbridge.
A specialist detection, identification and monitoring (DIM) team from West Yorkshire Fire and Rescue Service is also working alongside them.
It has been confirmed that two of the 27 pupils affected were taken to hospital, the rest were treated at the scene by ambulance staff.
We are working with colleagues from North Yorkshire Police and Yorkshire Ambulance Service at this time to determine what has caused this occurrence. Further information will follow as we have it.
Emergency services remain at the scene at Outwood Academy School on Clotherholme Road in Ripon.
Police were called at around 12.30pm - its is believed the incident happened sometime between 11am and 11.30am. A number of students had collapsed at the school and a number of others were taken ill. It is reported 27 students have been affected, all 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.
It has been confirmed that 27 pupils and one teacher have collapsed at Outwood Academy School in Ripon. The school has been evacuated and pupils sent to nearby Ripon Community College. Emergency services remain at the scene.
Emergency services are currently in attendance at the Outwood Academy School on Clotherholme Road in Ripon.
Police received reports at around 12.30 today that several students had collapsed at the school. A number have been taken to hospital.
A spokeswoman for North Yorkshire Police said the safety of all the students and staff at the school was paramount and officers were working with the fire and ambulance services to establish what has caused the students to collapse.
Police investigating a serious assault on a teenager in Ripon have charged a 22-year-old local man with causing grievous bodily harm. He will appear at Harrogate Magistrates Court later.
The victim, who is 18 and from Ripon, remains in a critical condition in hospital. Anyone who witnessed the incident on Quarry Moor Lane in the early hours of Sunday October 11th , is urged to contact the Harrogate Serious Crime Team on 101.
Police are investigating an allegation of a serious assault that has left a young man in a critical condition in hospital.
The incident happened on the night of Saturday 10 October 2015 on Quarry Moor Lane, Ripon.
It is alleged that the victim, an 18-year-old-man from Ripon, was attacked and as a result was taken to Leeds General Infirmary where he is in a critical condition.
The incident was reported to police earlier today, Monday 12 October 2015, and officers have secured the scene on Quarry Moor Lane to carry out a full investigation.
Watch the moment a mother and son are reunited after 53 years apart. Ann Lowery was forced to give her son Gary Collins up for adoption. But Gary tracked down his mother - who lives in Ripon - after a decade-long search - and flew in from Australia at the weekend.