The numbers. Among moderately advantaged schools, for example, public schools spent $10,932 per student on average in 2012, the most recent data available. Yet, to achieve similar results, Catholic schools spent an extra $588 per student and independent schools spent $1389 per student more, much of which comes from school fees. Among the most advantages schools, the average spend per student was up to $22,000 in private schools, more than double that spent on similar public school students. When looking at all schools across the state, the excess money spent on students who achieve the same results as their cheaper public school equivalents was $520 million in the Catholic system and $2.77 billion among independents.

Mr Bonnor, a former principal and public education advocate, argues these schools could "almost certainly" achieve the same academic outcomes without that additional money. "If there's no extra funding coming from governments, we need to take a serious look at where money is spent," Mr Bonnor said. "If it's not being spent efficiently and achieving results, it's perfectly legitimate to ask whether it should be redistributed to schools where it will have a greater impact." He argues, if $3.3 billion was diverted to our poorest schools, "Australia's worrying achievement gaps would diminish". The executive director of the Association of Independent Schools of NSW, Geoff Newcombe, would not respond to Fairfax Media's questions except to say "analysis of this type is ideologically driven and has no useful educational purpose". A spokesman for the federal education minister, Christopher Pyne, said increased money was allocated to disadvantaged schools under the needs-based model introduced in 2014.

Tim Hawkes, the headmaster of The King's School, published an article on his website last week in defence of spending money on a private education. "Most parents I speak to are looking for a great exam performance in year 12. But, this is only part of what they are looking for," he said. "They are also wanting a school that pays a lot of attention to values, that advances a faith position, that has a strong co-curricular offering, that offers boarding, that has strong accountability." Mr Bonnor said the argument that private schools were focused on "the whole child" was insulting to public schools. "All schools educate the whole child," he said. "All schools provide those experiences which enrich the lives of our kids. To say that this only happens in high-fee private schools is frankly nonsense." Asked whether money should be transferred to struggling schools, Education Minister Adrian Piccoli noted NSW was the first state to sign up to the Gonski needs-based funding model.

"We strongly support a funding system that provides resources to students according to need, regardless of school sector," he said.