Two crews from the Worksop station were called to the Prospect Hill Infant and Junior School around 8.30am after school staff attempted to extinguish the blaze.
A spokesman for Nottinghamshire Fire and Rescue was quick to dispel rumours the boiler had exploded, but did confirm the fire had started in the boiler room.
A smoke alarm alerted people to the fire and the school was immediately evacuated.
School staff initially used a fire extinguisher, which put out most of the fire.
The firefighters then went in with a hose reel jet and thermal imaging camera.
One man was given first aid by firefighters at the scene for smoke inhalation and he sustained some minor burns.
The fire was fully put out by 9am.