Sixty-four pupils and staff had to be evacuated from a boarding house during a fire at Hertfordshire school.
About 40 firefighters were called to Bishop's Stortford College on Maze Green Road at 03:00 BST.
A school spokeswoman confirmed the roof of Robert Pearce House had been destroyed in the blaze.
All pupils and staff got out of the building within a few minutes and no-one was injured, the school spokeswoman added.
Former pupil Stephanie Howard-Smith, who lives nearby, said: "I'm absolutely stunned. I could see the flames getting higher, and suddenly you could hear the roof falling in and cracking sounds. It's still smouldering now."
A fire service spokeswoman said six engines were still at the scene and an aerial ladder was being used to deal with a collapsed roof. The cause of the fire is not yet known.
The spokeswoman for Bishop's Stortford College, which offers day and boarding schools for four to 18-year-olds, said the fire appeared to have started in the roof.
The prep school and senior school had been closed as a result of the fire, but the school hoped to be operating "as near normally as possible" on Wednesday, she said.