UPDATE 5.30pm: Police are investigating a deliberately lit fire that ripped through a historic suburban primary school late last night.
Firefighters and police officers were called to Mt Lawley Primary School on Second Avenue at 11.57pm.
The fire has caused extensive damage to the school, affecting about 80 per cent of a building, according to police.
"The structural integrity of the building is a concern and engineers will meet this morning along with Arson Squad officers at the scene," police said.
"The investigation is ongoing."
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The fire led to road closures in Mt Lawley and motorists were urged to avoid Second and Third avenues between Beaufort and Clifton streets.
Early this morning, the Fire and Emergency Services Authority said that as a result of the fire, potentially dangerous smoke had become a problem in the area. It later updated its warning to "potentially irritating".
Although there was no direct fire threat to nearby homes, residents within 100m of the school were asked to remain inside, keep their doors and windows closed, and turn off their air-conditioners.
"People should stay away from the area. If you are already in the area take advice from emergency services personnel," FESA said.
"The Department of Health has advised that people with medical concerns should contact their local doctor or call Health Direct on 1800 022 222.
"ChemCentre and the Department of Environment and Conservation's Pollution Response Unit are conducting air-monitoring and analysis."
Career and volunteer Fire and Rescue Service and Bush Fire Service firefighters as well as a taskforce from Perth tackled the blaze.
About 50 firefighters were at the school at the height of the fire.
A meeting at Mt Lawley Senior High School was arranged this morning for parents and community members with an interest in the school.
The primary school's principal, Don Barba, said it was a very emotional time for those who had been involved with the school.
He said it was OK for people to grieve because it was "a very special place for our students".
Mt Lawley Primary School, formerly known as Inglewood State School, celebrated its centenary in March.
The school has "an exceptional history and continues to inspire and educate students today", according to its website.
The close relationship between the local community and the school is obvious in a statement by Mr Barba on the school's website: "We recognise that it takes all of us working together to make a school. Students benefit when there is a close relationship between the school, home and the community."
Elizabeth Hanjina was in tears as she surveyed the damage with son Alexander, who is in Year 1 at the school.
"It's such a fantastic school and a great community," Ms Hanjina said.
"I think everyone will be devastated."
FESA incident controller Darren Martin said the biggest challenge for firefighters had been how rapidly the fire moved through the building because of the large amount of timber and other quick-burning materials in the heritage building.
About 80 per cent of the school's buildings had been destroyed and the extent of the damage hampered efforts to put out the blaze and investigate what caused the fire.
"We have large amounts of building material on top of heated areas and that is a challenge to firefighters," he said.
Mr Martin said he was waiting for advice from structural engineers about the stability of the building and the safety for crews trying to get inside.
Police and fire investigators had not yet been able to determine exactly where or how the fire started.
He said an insurance assessor had been unable to give firefighters an estimate of the cost of the damage.
Mr Martin said two small pockets of asbestos had been identified, but Department of Environment and Conservation experts had provided "favourable information that it was contained to that small area".
Students will begin two weeks of holidays tomorrow, but the question remains: what will happen when they return?Anyone with information about the fire is urged to contact Crime Stoppers on 1800 333 000.