Expensive private schools were outperformed by public and Catholic schools in the HSC when you consider students from similar backgrounds, which shows paying for an education does not necessarily deliver better marks. 

A study found that among schools with families from the highest social and educational backgrounds, public schools including selective schools had the highest rate of HSC marks above 90 in 2013.

Out of the 60 most advantaged schools in the state, public schools scored above 90 in 38 per cent of their exams, on average, while the rate was 26 per cent in private schools.

Researchers Chris Bonnor and Bernie Shepherd found public and Catholic schools also had better scores than private schools among moderately advantaged schools, even when you take the state's competitive selective schools out of the equation.

This year's HSC results are released on Wednesday and Mr Bonnor is urging parents to keep in mind that a top rank does not guarantee a school is providing the highest quality or most innovative education.

"Schools in that top 100 that dine out on the stories of their high achievers need to take a cold shower and reflect carefully on the claims they're making," he said. "The heads of the schools that do really well almost always attribute their success to something the school is doing, but we don't know how much difference is being made by schools. The substantial contribution to their success is the capacity and background of the kids they enrol."

Almost 90 per cent of the schools which topped the HSC last year were also the most advantaged schools in NSW, showing social class is a far stronger indicator of how a school will rank than the quality of teaching.

James Ruse Agricultural High School, which has long been the top academic high school in the state, has the second most advantaged students in the state when determined by the education and occupation of its students' parents.

The highest-performing comprehensive public school, Killara High School on Sydney's north shore, also has more educated families than any other non-selective state school.

 

Helen Proctor, an education researcher from the University of Sydney, said it was "extraordinary" the link between parents' occupation and education and their children's successes was almost without exception.

"There's a really widespread belief among some members of the community that their kids will do better academically at a private school but it seems not to be the case," she said. "If you're just looking at academic results, it probably isn't worth paying all that money for an elite private school."

But, she added, most parents who choose high-fee private schools were not fixated on academic results.

"They say they're looking for an all-rounded experience, so they're looking for a peer group and opportunities to use all those fabulous resources and facilities," Dr Proctor said.

The executive director of the Association of Independent Schools of NSW, Geoff Newcombe, said "getting your money's worth" meant different things to different parents.

"Some will be totally focused on their child getting into medicine, while others will be looking for extracurricular opportunities or involvement in Saturday sport," he said.

Dan White, the executive director of Catholic schools in the Archdiocese of Sydney, said it was important to celebrate schools that record strong student growth and not just those with the highest raw marks.

"We look at schools like Trinity at Auburn and All Saints at Casula and what those students are achieving considering the literacy and language levels they had in primary school is just extraordinary," Dr White said. "These are schools that are punching above their weight."