LONDON, United Kingdom (AFP) – Some 40 students fell ill at a school in northern England Wednesday, leaving emergency services scrambling to find out what caused the incident.

It occurred at Outwood Academy School, in the historic town of Ripon, when a wave of students reported feeling faint and nauseous, some during an Armistice Day ceremony in the school hall.

The local Yorkshire Ambulance Service said as many as 40 students were being treated.

“Yorkshire Ambulance Service received an emergency call from Outwood Academy School in Ripon at 11:30am (1130 GMT) this morning to reports that a number of students had fallen ill suddenly with nausea and feeling faint,” it said in a statement.

“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.”

Police and fire services are investigating the cause of the incident. The Daily Mail newspaper reported that it may be linked to a gas leak and that a teacher had also been affected.

A parent of one of the students at the school, who did not want to be named, said some of the pupils had fallen ill during an assembly to mark Armistice Day — the anniversary of the end of World War I.

“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,” the man said.

Outwood, located near the picturesque Yorkshire Dales national park, has over 600 pupils aged between 11 and 18.

The academy is part of the Outwood education trust, which has thousands of students.