Western Australia's largest public high school has been forced to close after asbestos residue was discovered during demolition work.

Parents were asked to keep students of Willetton Senior High School in Perth at home today while tests are carried out.

The school has since advised parents it will remain closed for the rest of the week and classes will be conducted online.

The Education Department notified parents of the closure late on Monday via SMS and email.

About 1,800 students are enrolled at the school and they had been due to return for the start of term three.

Department spokesman David Axworthy said while the residue appears to be contained to one storeroom, teachers yesterday said they may have seen similar materials on other parts of the premises.

He told ABC Local Radio the entire school is now being checked to ensure students are kept safe.

"We won't be putting kids at risk, that's the key thing, but secondly we also need to make sure that the children have their educational program continued as soon as possible, and it may be in a different classroom, but it'll continue," he said.

"We've known Willetton is an old school and has been scheduled for major work as part of the rebuilding program.

"The buildings that are to be demolished later in the year are routinely monitored and checked so during that routine inspection they found some broken ceiling tiles and other residue ... that contained asbestos.

"At that stage, this was an issue restricted to a storeroom in a part of a building."

Mr Axworthy said the school was still scheduled to open as per normal until staff alerted the department to other areas of concern.

"When the staff came back yesterday for a planning day, some of the staff who saw pictures of the small residue in the room said: 'Hey, I think there may be something like that in another part of the school as well,'" he said.

"When they identified that we said we need to check the school."

'Not happy': Education Department informed at 11th hour

Mr Axworthy said contractors from Building Management and Works, which is run by the Department of Finance, have been on the site for the past two weeks.

He said it was unacceptable the Education Department was not told of the problem sooner.

"I am not at all happy with the fact that I only found out at five o'clock last night that our biggest high school has a potential problem," he said.

"But that's an issue that you'd need to take up with Building Management and Works."

Finance Minister Dean Nalder, whose portfolio oversees Building Management and Works, was contacted for comment.

Mr Axworthy said virtually every building in WA that had work done on it through the 50s, 60s, 70s and 80s contained asbestos.

He said there is a register for buildings containing asbestos.

Opposition queries asbestos action plan

Opposition Leader Mark McGowan said it was disturbing the issue did not come to light until the end of the school holidays.

"Why is it that our largest high school is not going to have students because of problems surrounding asbestos?" he said.

"The Government should have determined that there was a problem around this over the course of the last two weeks of school holidays.

"There shouldn't now be a problem now that students are coming back to school.

"The Government should've known about this, should have dealt with it and should be more worried about this than they are about their taxpayer-funded advertising campaign, which clearly has the Minister's attention.

"It is the biggest high school in the state; you'd think they'd have been on top of it before it got to this stage."

Education Minister Peter Collier said was confident the department had done the best they could in the circumstances.

"The decision to close the school occurred just after 5:30pm," he said.

"As soon as physically possible, the school notified the parents.

"It was done for the safety and welfare of the students."

He said only 12 students turned up to school today.

Teachers will work from North Lake Senior High School for the rest of the week, where they will connect to students over the internet so classes can continue.