East Sussex Fire and Rescue Service was called to reports of a fire at Buckswood School in Guestling, just outside Hastings at 23.45 on 13 September.
A single storey building measuring approximately 40 metres by 40 metres was alight.
Firefighters used six breathing apparatus, hose reel jets and a thermal imaging camera while tackling the fire.
Three members of the public suffered smoke inhalation and were left in the care of the ambulance service.
An investigation found the fire was accidental.