A Brisbane primary school has been shut down and a crime scene declared after a "significant" fire destroyed several classrooms.

Key points: A blaze broke out on the top level and roof of a two-storey building at the school just after 7:00pm on Friday

A blaze broke out on the top level and roof of a two-storey building at the school just after 7:00pm on Friday More than a third of the school's 500 students from prep to year four will be affected by the loss of their classrooms

More than a third of the school's 500 students from prep to year four will be affected by the loss of their classrooms The school will be closed on Monday while investigations and clean-up operations are carried out

The blaze gutted eight classrooms that were used by about 200 students from prep to year four.

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Police and fire investigators have returned to Upper Mount Gravatt State School this morning to investigate the blaze.

A blaze broke out on the top level and roof of a two-storey building just after 7:00pm on Friday and more than 10 fire crews worked quickly to bring the flames under control.

Police Inspector Geoff Acreman said the fire was being treated as suspicious.

"There were multiple intruder alarms in the classrooms where the fire has later been found," he said.

"It's going to be investigated quite intensively."

Inspector Acreman said it was devastating for staff and students.

"If it was deliberately lit, it's a very sad event … people who do this sort of thing, they have no regard to the amount of people that it affects, especially the children," Inspector Acreman said.

"The work the teachers actually put in or commit to these students, it goes up in flames as well."

Melinda Dare's two daughters are in prep and Year 2 and said her children had lost everything.

"That's their home … why would you go and ruin little kids places that they go and learn?" Ms Dare said.

"These teachers have worked so hard to accumulate all of those resources and it's just gone.

"It's terrible, just awful — it is what it is and we've just got to move on and just help support the kids and just hope the department gives some resources to the school, like counselling for the kids."

Melinda Dare's two daughters are in prep and Year 2 at the school. ( ABC News: )

Building to be demolished

The Department of Education said more than a third of the school's 500 students would be affected.

Deputy director-general of corporate services Jeff Hunt said the building would have to be demolished.

"I think any loss of a school building is a devastating thing for school communities and we'll work with the school," he said.

"We've already put plans in place to make sure that buildings are coming on to the site next week to assist the school to get back to normal.

"We want kids to get back to teaching and learning as quickly as possible so that their learning journey isn't disrupted."

Mr Hunt said it was not yet clear what set off the security alarms.

"Heat and motion sensors were triggered and the alarm goes through the State Government security," he said.

"We're waiting for the forensic teams to do their work … I think it's too early to say what the exact cause is."

School principal Derek Brady described it a "tragic event" for his "wonderful" school.

"This is very sad news for our school community and I ask that we all support each other as we deal with this," Mr Brady said in a statement.

The school is expected to be closed for at least Monday while investigators determine whether the fire was deliberately lit.