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
It had been reported that four domain names which appeared to support Clive Lewis for Labour leader were registered in June last year.
The EU flag symbolises unity between European countries.
Shadow business secretary Rebecca Long-Bailey has called for "immediate" relief to prevent thousands of companies going out of business.