More than 250 public schools have been closed across NSW as teachers demanding "more than thanks" go on strike for 24 hours over staff shortages and a dispute over pay.
About 15,000 teachers took part in the rally outside NSW Parliament in Sydney on Wednesday, while thousands more protested regionally.
Greenacre Public School principal Melissa Proctor said she could never find enough teachers for the almost 800 students at her school in Sydney's southwest.
Specialist teachers were teaching subjects they weren't qualified for amid a staffing shortage that was getting worse.
"Amazing teachers are leaving our profession because they can't cope with the ever increasing demand of the workload and the impact that has on their own families and wellbeing," she said.
Ms Proctor said the last few years have been the worst in her 25-year career as a teacher and principal.
Braidwood Central School teacher Alisa Stephens, daughter of a public school teacher and married to one as well, said the role has changed and teachers are now expected to do much more than their pay accounts for.
"Teaching is a job that is impossible to do in our paid hours, and the government knows that," she said.
"Unfortunately for us, our families know that too."
Along with nurses, paramedics and public transport staff, teachers want a pay rise above the 2.5 per cent cap on NSW public sector wage rises that has stood since 2011.
They also want pay for some of the extra time they spend on lesson planning and other paperwork.
The NSW Education Department listed 264 schools as "not operating" on Wednesday, with parents urged to keep children home as there was minimal supervision at schools that were open.
Education Minister Sarah Mitchell said the disruption to students and parents caused by the strike was "frustrating and disappointing".
NSW Teachers Federation president Angelo Gavrielatos said children were having their learning disrupted every day due to a lack of teachers causing classes to merge.
"To say their learning is being disrupted is a massive understatement, and yet the premier and the minister want to lecture us about disruption," he said.
Teacher shortages will get worse if their pay doesn't get better, Mr Gavrielatos said.
Ms Mitchell asked the union to cancel the strike on Tuesday, telling them to wait until the June 21 state budget when the government will address public sector demands for increased pay.
Labor education spokeswoman Prue Car said it was silly of the government to keep hinting at relief in the budget.
"Just tell us what it is so we can avoid this disruption and actually get some solutions for the key workers that got families through the hell of the last two years," Ms Car said.
Uralla Central School principal Michael Rathborne said the budget promise was akin to a pat on the head, and accused Ms Mitchell of denying there was a problem with staffing in the state's schools.
"It's not good enough," he said.
"Things have never been worse."
Staff absences led to at least one class being cancelled every day at his school, and he and his deputy principal regularly had to teach classes themselves.
His search for a school counsellor, after seven churned through the school in eight years, was on its third round of recruiting.
Ms Car declined to say what wages would look like if Labor wins the March election, but said teachers deserve more than their current rates.
Public sector workforce wage rises amount to billions of dollars and the government has competing priorities that need to be balanced, Ms Mitchell told Sydney radio 2GB.
"These are major decisions that should actually be made as part of a proper budget process, not in response to union demands," she said.