Up to 30 students and some teachers were treated for smoke inhalation after a smoke bomb was set off in a school.
Emergency services were called to Pobalscoil na Tríonóide in Youghal, Co Cork at 2.20pm.
Gardaí described the incident as a "schoolboy prank gone wrong".
The large secondary school on the outskirts of the town was cleared as a precaution.
The injured were examined by HSE ambulance staff in the grounds of the school.
There are no reports of serious injuries and no-one was hospitalised.