A neighbour's early morning call to the fire service after smelling burning coming from the Nicholas Hamond Academy in Swaffham could have saved the whole building from going up in flames.
Firefighters were called to the school on Brandon Road in Swaffham at about 4.40am today, after the neighbour alerted the brigade.
The school has been closed today following the fire which caused damage to the science block, and left a gaping hole in the roof, which collapsed.
An office and a storage room were destroyed in the blaze, including IT equipment and science resources and books.
Six classrooms suffered smoke damage and the school was left without gas and electricity.
The secondary school and sixth form in Brandon Road has about 600 pupils on roll.
Principal Cheryl Hill said only about a third – those in exam groups – would be allowed back to school tomorrow and Friday.
She said the science block, which dates back more than 50 years, was in the process of being re-roofed.
As next week is half-term, she said they hoped they could repair the damage in time for the school week starting February 22.
Afterwards, Joe Warns, King's Lynn fire station manager, said: 'We are relieved that we got a call when we did, as the fire could have spread to the whole school.
'The fire was well alight when crews arrived and there were flames coming through the roof. Crews had to break down the door to get inside.
'Our investigation into the cause of the fire at present is centring on the work being carried out on the roof by workers the day before, when gas torches were being used.
'Our crews were on scene more than five hours until 10am. Crews from Swaffham, Watton and King's Lynn attended.'
Ms Hill, who took over the school in 2012, added: 'It's too early to say how much the damage will cost to repair, or what's been lost. You cannot get that close to the building. It's a blessing no-one was in the block at the time and we would like to thank the person who called the fire service. 'Children won't be able to have lessons in the six damaged classrooms until they're cleaned.'
Are you the mystery caller? Email david.bale2@archant.co.uk
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules here