Firefighters have brought a blaze at Christchurch's Beckenham School under control and managed to save the under-threat school hall.
Four crews were sent to the Sandwich Rd school shortly after noon and two more were called in as back-up.
Residents described thick black smoke and flames shooting out of the roof and one couple said they heard an explosion.
Fire Service acting assistant area commander Mike Bowden said the fire was believed to have started outside the school's library and spread quickly into both the library and an adjoining multi-purpose room, used by an after-school care programme.
Specialist fire investigators and police would be called in to determine the cause.
''School fires at this time of year have a natural suspicion. We're quite hopeful there is some security footage around, but it's way to early yet to tell how it started,'' he said.
Bowden said the flames and smoke were visible from several kilometres away.
The strong nor'west wind has driven the fire through the roof and spread from there, he said.
''Our main strategy was to stop the fire spreading into the hall, which we have been successful in doing.''
Firefighters would likely remain at the scene for the rest of the day.
Central station officer Omar Yusuf said they were now dampening down and searching for hot spots.
''We've put thousands of litres of water on it, but still on a hot day with strong wind it's still a risk until it's stone cold.''
Mark Collins, who lives opposite the school, said the fire quickly grew, fanned by the strong, warm winds.
"It was very fast. We just watched it getting bigger and bigger and bigger,'' he said.
"The wind was blowing the flames into the building itself."
His wife, Colleen Ford, went across to the school to check on the Bosco holiday programme, which uses the multi-purpose room.
There were about 30 children on the programme but they had been on an outing when the fire started. They arrived back by bus to find their hall on fire.
School principal Sandy Hastings said she got the call about the fire while she was at the airport.
''It's gutting,'' she said.
''It's great that it's in the holidays, but you never want to see your school on fire. It's something everyone in education dreads.
''My biggest concern is we've now lost two buildings that may be deemed by the Ministry of Education not to be a priority because they aren't classrooms.
''A library is vital to childrens' learning as are other learning spaces. We'll wait and see.''
Hastings said the school would work with Bosco to find them another facility to use.
''It's important they keep going.''
- The Press