"Access to university should be fair and equitable," Mr Piccoli told Fairfax Media. "Any scheme that gives some students an unfair advantage is unacceptable and I will be discussing this with the federal government, which is responsible for universities."

As revealed by Fairfax Media, boys from the elite college are the only school students in the state to benefit from a pilot of the Diploma of Tertiary Preparation, which is aimed at teenagers with predictive tertiary admissions ranks of between 55 and 70 per cent.

Opposition Leader Luke Foley says it puts the prestige of NSW's internationally respected HSC at risk by allowing students at the "top-end" GPS school to "pay big bucks" for a diploma shortcut into university.

On Monday morning, Mr Foley challenged Mr Piccoli to "cut off once and for all any arrangements that allow people to pay to get around the HSC and gain access to university via a fast-tracked diploma".

"We have a federal Coalition government intent on linking university admission to wealth, more and more, and these are the sorts of arrangements that flow from the Abbott government's reform of our higher education sector," Mr Foley said.

"I think there's a role here for the state government to step in as the custodian and protector of our Higher School Certificate."

Concern among University of Sydney staff – who were worried about preserving equitable access to degree courses – has led its academic board to shut down this pathway for school-age students. On March 25, the board resolved that any diploma applicants for its degree courses who were younger than 21 would still need to have the HSC or equivalent qualification.

Eight Scots students took a 17-week intensive course for the diploma in 2014. Six were offered places in selected degree courses at the university this year. Four enrolled and one deferred.

But when the academic board took action last month, it was unaware that Scots was continuing the pilot with another 11 boys this year – or that the school had a contract with Sydney Learning, a wholly-owned subsidiary of the university which devised the diploma course.

Scots principal Ian Lambert pointed out that contract when responding to questions from Fairfax Media. The board's chairman, Associate Professor Peter McCallum, said: "It goes without saying that any undertaking given to students will be honoured."

Sydney Learning normally charges $12,000 for the diploma, typically aimed at mature-age students who missed out or "bombed out" on the HSC. It has not disclosed how much it is charging Scots, which is absorbing the cost in its school fees.

"I'm glad there's been a backlash at the University of Sydney," Mr Foley said. But he is worried about the "rampant commercialisation" of universities and "here's a pilot that any other entrepreneurial university may seek to progress with their own contractual arrangements with other elite schools".

In minutes and agenda records of the University of Sydney's academic board and admission committee obtained by Fairfax Media, members worried that:

• Students who took the HSC pathway could miss out on entry to certain courses if they became full.
• The Scots pilot was not mentioned in the original proposal for the diploma.
• The diploma was advertised to year 11 and 12 students as an alternative to the HSC and "concern was noted about possible implications for the university's reputation".
• The diploma was conducted over 17 weeks at Scots, while it had been approved under the understanding it was a one-year full-time course.

Ann Brewer, the chief executive of Sydney Learning and the university's Centre for Continuing Education, helped Scots establish the diploma pilot. She also helped establish the school's Research Centre and a customised master's course for its teachers. Like the boys in the diploma course, the teachers could remain on the school campus while completing their master's degree.

Scots students who passed the Diploma of Tertiary Preparation with an average of 65 per cent or more were guaranteed places at the university but in a restricted range of bachelor courses – liberal arts and science, health sciences, animal and veterinary bioscience, visual arts, music and oral health – or in the diploma of law.

Dr Lambert defended the program, saying: "The literature on boys' learning is clear – the HSC is a great matriculation pathway for the top 20 per cent of students, but has mixed opportunities, particularly for boys, in the middle rank of learners."

These students could not access "special needs or talent pathways", so the diploma was designed with them in mind.

There was no "exclusivity" for Scots and anyone could apply to do the diploma "alongside other HSC subjects, for example, if a student were enrolled in the TAFE HSC equivalent", he said.

Associate Professor McCallum acknowledged concerns about the pilot were raised but told Fairfax Media: "There's nothing in this program we want to hide. It was an honest attempt to widen access to the university. We did a pilot and I'm fully aware of the perception issues you raise. But sometimes you have to forge ahead and pursue the objective to widen access."

He could not say why Scots was chosen for the pilot but he understood that other schools, including disadvantaged schools, had been approached.

He pointed out that such courses were not government-funded, so other schools might have been unable to meet the costs.

But the academic board acted because it did not want to lure students away from the HSC or to leave school early to do a diploma "against their interests", Associate Professor McCallum said.

Mr Foley, when asked if he would be less worried if the diploma had been offered to all school students, or if the pilot was conducted at a poorer school, said: "It would still worry me … because what we're seeing is people having to pay for a diploma.

"But secondly, and most importantly, the prestige of our internationally recognised HSC is in danger here."