Loud explosions were heard at the scene of the Selsey Academy fire before it was engulfed by flames.
More than 70 firefighters are tackling the fire at the school in School Lane.
The blaze started at around 8am.
Scores of residents in Selsey watched in disbelief as the fire took hold this morning (Sunday, August 21).
Hampshire Fire and Rescue Service has sent two appliance to help West Sussex Fire and Rescue Service and more crews are on standby.
The fire service is advising residents to stay away from the smoke and close their windows.
A spokesman for West Sussex Fire and Rescue Service said the blaze started on the roof of the school and more than 75 per cent of the building was engulfed in flames.
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