Students are bundled into composite classes or left to their own devices thousands of times a day due to a shortage of casual teachers in NSW.
A survey of public schools found more than 9800 lessons were without a teacher each day, with 87 per cent of schools impacted.
Those worst-affected were rural schools, those in Sydney's south and west, and schools that operated for a specific purpose.
In high schools, students in nearly 30 per cent of uncovered classes were left to their own devices with minimal supervision, the state government said on Tuesday.
The teachers' union said the data came as "no surprise".
"The teacher shortage in NSW public schools is a direct consequence of the former government's wage cap that artificially suppressed teachers' pay," NSW Teachers Federation acting president Henry Rajendra said.
"The wage cap made the profession less attractive."
But the education minister's crusade to improve classrooms hit a speed bump on Tuesday when she was unable to pinpoint a goal in reducing permanent teacher vacancies.
Officials on Tuesday confirmed 2247 positions were vacant on September 25.
In opposition, Prue Car made hay of the rising number of vacancies, criticising then-minister Sarah Mitchell for dismissing it as "a bit of a beat-up" because it can represent those on leave.
But pressed on what she wanted the number to be, the education minister deflected during a budget estimates hearing.
After being challenged repeatedly for a target, Ms Car said the government would be transparent about shortages going forward.
Labor made several election pledges to improve the plight of teachers and recently struck a deal to significantly lift salaries.
As of this term, public teachers at the bottom and top of the pay scale are paid best in NSW while administration duties have also been wound back.
The deal also lifted casual teacher pay rates.
But Ms Mitchell said the government had failed to articulate a plan for reducing teacher vacancies, which had not gone down since Labor took power.
"The minister is relying on her pay increase to assume that that will help bring people into the profession," she told reporters.
Ms Car earlier dismissed opposition concerns of a school infrastructure black hole after it was revealed the $760 million allocated to build 14 new schools would only cover planning costs in some cases.
The department's infrastructure boss told budget estimates that a greenfield, 1000-pupil primary school cost about $70 million while a greenfield, 2000-pupil high school cost about huyggf $200 million.
"Even if you take the planning projects out, it's still not enough money," Ms Mitchell said.
"What we've seen is a minister that's all political spin, no substance and parents of this state should be really concerned."
