The property had been refurbished within the past year.
@999London
Nicola Lown of London Fire Brigade was at the scene. She said: "The ground floor partially collapsed into the basement leading to hazardous conditions.
"Two fire rescue units were also on the scene and their equipment was used to shore up the ground floor.
"This enabled crews to wear breathing apparatus to gain better access to the fire in the basement."
At least 15 fire engines attended the scene after the brigade was called minutes before midnight. The fire was brought under control shortly before 7am today.
Fire crews from Stoke Newington, Tottenham, Homerton, Hornsey, Holloway fire stations and other surrounding fire stations were at the scene.
The cause of the fire is under investigation but a Metropolitan Police source said it was believed to have been started by an electrical fault. Another source said the fire started in the boiler room.
A police spokesman said: "The school was empty at the time and evacuation was not required.
"At this stage it is believed to have started from an electrical fault.
"Cordons remain in place although they are likely to reduce over the day. Several residential houses have been effected by electrical failure however the engineers are working on restoring power."
