The blaze damaged a large shed at Chelfham Mill School at Chelfham, near Barnstaple.
Fire crews from Barnstaple and Braunton were called to the incident at around 8pm last night. A water foam carrier and additional staff from South Molton were also requested.
Fire fighters wore breathing apparatus and used two hose reels and a jet to extinguish the blaze.
The large storage shed attached to the school was severely damaged, said firefighters.
Devon and Somerset Fire and Rescue Service said the fire was believed to be arson.
Police also attended the fire.
Chelfham Mill School is described as an independent special school.
According to Ofsted the school has 37 pupils aged seven to 19.
It educates pupils with emotional, behavioural and social difficulties.
Fees range from £29,361 for day pupils to £53,775 for boarders.
In September 2008 the school was evacuated after a fire started in a bedroom. Nobody was injured.