"The IB cohort is very small and select,"a UAC spokesperson said."The results we receive from the IB are very coarse which makes differentiating between students,which is what a rank is trying to do,very difficult."

Changes to be introduced from 2022attempt to remedy this by making the conversions more granular:students identified as having a high 45 will still receive 99.95,but weaker 45s will yield 99.90. From 42 down there will be three sub-divisions for each IB score.

The new system will likely see 25 per cent of students increase their rank and 15 per cent receive a lower rank.

Why do schools offer the IB?

Antony Mayrhofer,who has worked with the IB for 30 years and co-ordinates its delivery at St Paul's Grammar School in western Sydney,believes the diploma's strong focus on critical and creative thinking,as well as conceptual understanding,are of greater value to students than content recall.

"A lot of schools introduce[the IB] because of the pedagogy;it strongly develops critical thinking skills,"he said."Others are drawn to the philosophy,which is very idealistic. And sometimes it comes from experience – principals move to a different school and want to introduce it."

Linda Emms,head of learning and teaching at MLC Sydney,said the IB was popular among students who wished to pursue overseas studies in the future,because it is recognised globally by universities. It also helps the school cater to international students who may have studied the IB overseas.

But schools face operational challenges in running the IB,such as advanced timetabling required to run two programs at once as well as teacher training. Schools must be authorised by the global International Baccalaureate Organisation to offer the diploma.

"Ensuring all of our staff are up to date on changes to the curriculum and training staff new to the IB program is a continuous cycle,"Ms Emms said.

Still,many private schools have chosen not to offer the IB. Jenny Allum,principal of SCEGGS Darlinghurst,said she thought the HSC was more comprehensive and courses were as high quality as the IB.

PLC Sydney principal Paul Burgis said other international programs,such as Cambridge courses,allowed greater flexibility and could be tailored to student needs."The IB expects schools to use its professional development program,and it is a unified,single system,"he said.

Which is the right fit for my student?

Mr Mayrhofer said 96 per cent of Australian IB students go on to university,and that the diploma's emphasis on university preparation means it is not suitable for all students.

He also thinks it comes down to whether students want a broad educational approach in their senior school studies,or if they would prefer to hone in on a topic or vocational area.

"The IB Diploma is deliberately broad-based,whereas the HSC is much more flexible – it can be broad or it can be narrow to suit a student who really wants to specialise,"he says.

"The HSC also offers the chance to refine subject selection through adding extension subjects and being able to drop a subject you don’t enjoy as much."

Some students are attracted to IB subjects such as psychology and global politics,which are not available in the HSC. But deterrents include the requirement to study a foreign language.

Others are driven by scaling. Students who are strong in the arts might prefer the IB because it treats those subjects the same as maths and science:a grade 7 in theatre or music is worth the same as a 7 in physics. The HSC,on the other hand,has historically tended to elevate students who perform well in extension mathematics,physics,chemistry and economics.

The IB workload can also be a source of stress for students. Teachers say it requires a different pattern of study to the HSC;students need to be prepared to commit to being organised over a two-year campaign.

Then there's the fail factor:whereas all students are awarded the HSC regardless of their marks,a student can fail to receive an IB diploma. Last year's pass rate was about 92 per cent. Students must score a minimum 24 points to receive the formal qualification and an ATAR equivalent.

But they must also score at least grade 4 in each of their higher level subjects. They cannot score a single grade 1,nor the bottom mark in either the essay or theory of knowledge subject. If they fail to meet those requirements,students can sit the IB exams up to three times and still receive a record of their results.

Will the IB become more widespread?

The IB diploma was offered by 15 NSW private schools in 2015;now it's about 20.

"We see new schools joining each year,or most years. The proportion of students taking it is growing as well,"Mayrhofer says. St Paul's and MLC,which have offered the IB for decades,now have about half their cohort take the diploma.

But most IB schools are in affluent areas – just two are west of Strathfield and two are south of Maroubra – and it is still not available in NSW public schools. Part of that problem is the price tag,which was costed by the NSW Department of Education at about $300,000 per school each year.

Mr Mayrhofer acknowledges there is a"feeling of exclusivity"around the course in NSW,although other Australian states are increasingly offering the IB in public schools. Queensland introduced the diploma to three selective and six comprehensive public schools in 2010.

Aninternal NSW Department of Education paper from 2017 recommended offering the IB in government schools to give students equity of access with private schools.

The proposal also said it would"help attract and retain families of bright students in the public school system"by offering subjects more advanced in depth and breadth.

But the paper was never published nor acted upon. The challenges of introducing the IB included difficulty in offering it in smaller high schools,high costs and the risk it"may devalue the HSC". There were also concerns its implementation could exclude students outside metropolitan areas.

"We do not currently support or endorse any of the International Baccalaureate programs in NSW public schools,"a NSW Department of Education spokesman said."The Higher School Certificate is a world-class qualification that is available to all NSW school students."