Devon and Cornwall Police asked anyone who lived in the Stockley Hamlet area to ensure their windows and doors were closed, and asked people to avoid the area if possible.
Sgt Tom Ottley said there was “significant damage to the entire school”.
“[It is] quite a significant sized blaze, fire have it under control, we don’t believe there’s any further risk to property or any casualties at the moment, so it’s just about trying to control the damage and assess the damage thereafter,” he added.
Ottley said police were in touch with the school and parents of its pupils would be contacted overnight to let them know about plans for the coming days.
He urged anyone who worked in the area to check for the latest information on Monday before travelling.