Chapelside Primary School, in Airdrie, could be closed for days after vandals broke in overnight.
They are believed to have set fire to the school's boiler room, leaving the primary without heating, lighting or electricity.
Education bosses are currently surveying the extent of the damage.
But they said it could be some days before the school is reopened, depending on the severity of the vandalism.
Coatbridge Police Office has also appealed for information into the fire in the school