Firefighters were called to the blaze, at the former Foyle College site on the Northland Road, at about 3.30 am this morning, Saturday, July 7.
Earlier police warned people living close to the former site to keep windows and doors closed due to asbestos in the building.
However, the fire service said it was at a lower level than they feared.
“We’re comfortable that there’s no impact to the public around asbestos,” Group Commander Alan O’Neill said.
The blaze was fully extinguished on Saturday morning.
The building has lain empty since the school moved to its new site on the Limavady Road in the city’s Waterside.
A fire at the site in April led to warnings about vandalism.
Sinn Féin MLA Karen Mullan said the fire started in the middle of the night.
“The school seems to be totally destroyed,” she said.
“There has been ongoing vandalism at the school for the last number of nights.
“On Tuesday night, I had been contacted about anti-social behaviour at the site and the breaking of windows.
“We contacted the school on Wednesday and the board of governors to ask for an immediate meeting.
“The earliest we could arrange the meeting was the start of next week.
“In the meantime, I had asked, particularly over the nights and weekends, that the school stepped up security,” she added.Detectives at Strand Road are treating the incident as arson.
Anyone with information about those behind the attack are asked to contact detectives on the 101 number.
Alternatively, information can be passed anonymously to Crimestoppers on freephone 0800 555 111.Tags: