Firefighters were called to The Weald at 7pm on Tuesday (March 28) after a blaze broke out in the ceramics classroom.
Smoke poured out the building, in Station Road, as six crews fought the flames, stopping the fire from spreading to other parts of the school.
Police said a 16-year-old girl was arrested on suspicion of arson.
The girl has been bailed until May 2, while enquiries continue.
Thirty people from neighbouring Billingshurst Leisure Centre were evacuated as a precautionary measure.
The classroom was 20 per cent damaged by fire, and 80 per cent damaged by heat and smoke.
The school was open to pupils the following day.
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