Four teenagers have been arrested in connection with the fire at the Darul Uloom School in Chislehurst, south-east London on Saturday night, Scotland Yard said.
Two 17-year-olds and two 18-year-old were arrested on suspicion of arson late yesterday and taken into custody.
Bromley Borough police officers are continuing to establish the full circumstances of the fire.
More than 100 pupils and staff at the Islamic boarding school had to be evacuated when the fire broke out.
More top news
Prince William and Kate have announced they are to become 'full-time' Royals as they move their family back to London from Norfolk.
Jobs for Americans, a strengthened military, new trade deals and support for police will shape Donald Trump's four years in government.
Donald Trump pledged to put "America first" as he addressed the nation after being sworn in as the 45th president of the United States.