Students could bring COVID-19 onto school campuses and spread it to classmates before they are required to do a rapid antigen test, NSW authorities have conceded.
Responding to criticism that some schools are yet to receive RATs, Premier Dominic Perrottet said the twice-weekly testing regime didn't have to begin on day one of the school term.
"There was never a requirement (the tests) be there on the first day," he told reporters in Sydney.
Education Minister Sarah Mitchell had earlier been "asking" all students and staff to test before arriving at school.
Six million RATs are being distributed to the state's 3000 schools to enable students and teachers to test themselves twice a week throughout February.
While most state school students begin on Tuesday, some private schools began classes on Thursday including Tamworth's Calrossy Anglican School.
The 1000-student K-12 college received 3000 RATs on Monday, allowing it to uncover a couple of asymptomatic cases in new boarders and students who arrived on Thursday.
Staff also conducted a RAT before arriving but other students, returning on Friday, won't have to test until Monday morning.
"There's no way we'd be able to ask parents, including some who live two or more hours away, to pick tests up before Friday," principal David Smith told AAP.
Instead, students will go home on Friday with two tests each and use one on Monday and Thursday mornings. Parents will notify the school where positive results occur. Extra tests bought by the school will allow daily testing of boarders.
"The big challenge will be how many staff and kids will we be without," Mr Smith said.
"We want to be safe and careful as we can - we're still doing what we can to cohort kids (with junior, middle and senior students kept separate)."
Elsewhere, parents at Goulburn High School were offered a RAT pick-up drive-through this week, ahead of classes beginning next week.
Meanwhile, authorities have reiterated the benefits of vaccination rates, with one in two children aged five to 15 years unvaccinated.
The uptake of third doses among adults is moving slowly - from 29 per cent to 36 per cent in the past week.
NSW Health Deputy Secretary Susan Pearce said about 100,000 vaccination bookings went begging at state-run clinics last week, suggesting "a perception in the community that Omicron is milder" was to blame.
"What we know is that to prevent severe disease associated with COVID, that booster is absolutely critical," she said.
About seven per cent of NSW adults are either unvaccinated or have had just a single dose. They made up 31 per cent of the deaths reported on Thursday.
Ten women and 19 men died, including two people in their 60s and 19 who were 80 or older.
The state recorded 17,316 cases in the 24 hours to 8pm on Wednesday, shifting the seven-day average below 20,000 cases for the first time since January 4.
Hospitals are treating 2722 COVID patients, down 72, while the number of intensive care COVID patients rose to 181. Of those, 72 are on ventilators.
Meanwhile, six out of 10 NSW intensive care nurses say they have "no intent of sticking around" once the Omicron outbreak subsides.
The NSW Nurses and Midwives' Association says there is a feeling of "despair" among ICU nurses in a recent survey.
"There are members who are telling us that they will get through this crisis, and then that's it," acting assistant general secretary Michael Whaites told AAP.
Elsewhere, 1073 first-year doctors began orientation on Thursday, including 162 in rural hospitals.
Australian Associated Press