Fire fighters have been called to Wandle Valley School in Carshalton
It is understood the fire, at the school in Welbeck Road, is now being brough under control.
Six fire engines and 35 fire fighters are at the scene.
It is understood that some foam mats caught fire.
A Sutton Council spokewoman said: "We understand there has been a fire at Wandle Valley School, which is of great concern to us.
"The fire has now been extinguished. The school has let us know that all children and staff are safe.
"Where the school has been able to contact parents, children have already been sent home. Support has been offered to the school and we are here to help."
“We will comment further as more details become available.”
More to follow.