Western Australia has recorded 19 new local Omicron cases as students at another Perth school are forced into two weeks' quarantine.
There is growing doubt over the sustainability of the state government's hardline COVID-19 rules after Atwell College became the fourth school exposure site in as many days.
A letter sent to parents on Thursday said an infected person had attended the Year 12 common room and multiple classes earlier in the week.
All Year 12 students will be required to isolate for 14 days.
It comes just days into the school year and after cases were detected at Corpus Christi College, Harrisdale Senior High School and Winterfold Primary School, plunging hundreds of students and teachers into 14-day quarantine.
Premier Mark McGowan has promised isolation periods will eventually be halved for COVID-infected people and their close contacts.
But despite growing backlash, the premier on Thursday insisted the rules would only change when the state reached a yet-to-be defined higher caseload.
"We'll eventually move to seven-day quarantining, and in certain circumstances people will still be able to go to school and go to work," he told ABC radio.
"But at this point in time, because we have very low case numbers, we take a precautionary approach.
"If you take a less precautionary approach, you enhance the spread, more people get it and so the cycle goes on. So whilst we get vaccination rates up, caution is the order of the day."
The Australian Medical Association called for the quarantine period to be reduced to seven days for close contacts at schools, adding that the state government should provide free rapid antigen tests for students and teachers.
"We can find no justifiable evidence-based reason, from an infection control and public health point of view, for the second week of quarantine for children," AMA WA president Mark Duncan-Smith said.
"It is doing harm to schooling and socialising, and taking parents out of the workforce."
Mr McGowan has also refused to say when the borders will reopen, having backflipped on a plan to reopen from February 5.
Qantas chair Richard Goyder and Wesfarmers chief executive Rob Scott are among the local business heavyweights who have publicly criticised the premier, saying they will relocate to the eastern states to escape his policy settings.
But Prime Minister Scott Morrison backed the decision to retain the border closures, which remain popular among many WA voters.
"That's the premier's call. He has to make that decision based on what he thinks his health system is ready to absorb," Mr Morrison told Perth radio 6PR.
The premier has repeatedly insisted WA's public health system is "strong and ready" for an inevitable surge in cases, despite suggestions to the contrary from frontline health workers and record levels of ambulance ramping.
More than 400 students have been ordered to isolate for two weeks across the Harrisdale and Corpus Christi schools.
Winterfold Primary School reopened on Thursday with replacement teachers, having kept its doors closed on Wednesday for deep cleaning.
Two teachers tested positive after attending a professional development day attended by about 55 staff, all of whom were ordered to quarantine.
About 27 children were also forced to isolate along with their close household contacts.
Three of the new Omicron cases reported on Thursday are not linked to known clusters, and several were infectious while in the community.
