The blaze broke out at around 1.30am this morning.
Gardaí are investigating an arson attack at a midlands attack.
Paddock National School is closed today, while the investigation is carried out.
Emergency services were called to the scene at around 1.30 this morning.
Crews from Mountrath and Mountmellick managed to bring the blaze under control and damage has been confined to one classroom - which is badly damaged.
Gardai are carrying out door to door enquiries and the scene is being examined.
Principal Bernadette Fitzpatrick is thanking the Gardai and Fire service for their quick response.
Anyone with any information is being asked to contact Portlaoise Garda Station.
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