Firefighters were called to King Charles School on Friday after a report that there was the smell of smoke in the main hall.
Two crews from Falmouth and one from Truro were called to the school on Western Terrace at 11.36am.
Pupils were evacuated to another building on the site while firefighters investigated the cause of the incident.
Crews using a thermal image camera and ladders found a defective fan in the ventilation system.
It was isolated by an engineer.
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