Emergency services, including several ambulance crews, were called to a school in Armagh on Friday.
Nine students were then taken to hospital after falling ill at the Southern Regional College.
A 999 call was received around lunchtime. The PSNI said officers received a report that "a number of pupils required medical attention".
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They added that enquiries are ongoing to establish the cause, Belfast Live reports.
All those affected were students at Markethill High School, who attend the college one day per week for vocational programmes.
It is understood that the pupils became unwell but the nature of their illness remains unclear. All of the students have since recovered.
A spokesman for the NI Ambulance Service (NIAS) said: "NIAS despatched two emergency crews, an intermediate care crew and two ambulance officers to the incident.
"Following assessment and initial treatment at the scene, nine patients were taken, by ambulance, to Craigavon Area Hospital."
Newry and Armagh SDLP MLA Justin McNulty said he had been in contact with the college after the incident.
He said: "Have spoken to the Chief Executive of the SRC following the news of the incident at the Armagh Campus. I’m wishing all the students who required medical attention a full recovery."
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