Four fire engines and around 25 firefighters were called to a fire at a school in Hampton Road in Twickenham.
Part of the ground floor toilet area in a two-storey school building and part of the roof were damaged by the fire.
Around 40 pupils and 15 members of staff left the building before the Brigade arrived. There were no reports of any injuries.
The Brigade was called at 1308 and the fire was under control at 1528. Fire crews from Twickenham, Southall and Richmond fire stations attended the scene.
The Brigade's Fire Investigators believe the cause of the fire was an electrical fault in an extractor fan.