Public schools across NSW will have their budgets slashed by up to $148 million in a move principals say will spiral decreasing enrolments and fuel further exodus from the government system.
But the state's education minister has fiercely defended the cuts, saying they have nothing to do with finding the money to pay for teacher pay rises.
Secondary Principals' Council president Craig Petersen said a 1.25 per cent budget cut for the calendar year would make it much harder for the public system to win back its declining share of students.
Enrolments have dropped nearly 25,000 in the past four years as more students shift to private education.
"It puts us in a challenging environment," Mr Petersen said on Wednesday.
"We don't have the resources to compete with some of our more affluent non-government partners."
He said the impacts would vary across the state, including planned classroom upgrades possibly being axed or outdoor play areas not going ahead.
Under the changes, delivered in a letter to public principals on Tuesday afternoon, deputy and assistant principals will be forced back into the classroom up to three days a week to fill teaching shortages
Education Minister Prue Car said the "necessary reset" reflected declining public school student numbers and an over-staffed executive cohort.
But she gave assurances core funding was not being reduced.
"This is just discretionary funding ... it's actually a completely separate kettle of fish," she said during a heated press conference.
Ms Car said the cuts would come from funds used previously to hire additional executives as the state grappled with a chronic teacher shortage.
But Mr Petersen said additional executive roles had been created to reduce the administrative burden on teachers and most head teachers and deputy principals were already teaching.
The Parents and Citizens Federation of NSW said the cuts would increase the burden on parents, carers and volunteers.
Education Department secretary Murat Dizdar noted the budget reduction could have been avoided if a deal had been struck with the federal government for more money.
NSW has been pushing for the Commonwealth's share of public school funding to be lifted to 25 per cent, from 20 per cent of the required level to meet students' needs.
States pay the remaining 80 per cent but NSW and most other jurisdictions are falling well short of meeting that target, leaving public schools short-changed by billions of dollars a year.
Federal Education Minister Jason Clare said public education should be parents' first choice and negotiations were continuing over a funding deal.
Opposition Leader Mark Speakman said principals had been blindsided by the decision and the timing was concerning.
"Now school principals are forced to look at cutting staff and programs to find savings," he said.
"The school year is well under way, meaning key decisions have already been made and people have already been employed for specific roles.
"This is not good enough and our kids deserve better."
