Around 100 pupils were evacuated from the building when the fire started in a void between the ground and first floor of a two storey school under construction.
London Fire Brigade said: “The Brigade was called at 0932 this morning to Carshalton Beeches primary school.
“Fire crews from Wallington, Sutton, Croydon and Purley fire stations are attending the scene. The cause of the fire is not known at this stage.”
The blaze was put out after two hours.