Take two schools in Sydney sitting the HSC.
Both are government schools, in a similar region, of a similar size and with almost identical levels of educational advantage.
But students at one are nearly five times as likely to receive a high score than the other.
This isn't a hypothetical scenario - but real-world results likely to be pored over by education experts in NSW following the release of last week's HSC results.
Examining the number of high scores awarded in the HSC (band 6 or E4) shows a system highly weighted to top-tier schools.
There were 588 NSW high schools that had more than 150 HSC course entries (we have left smaller schools out of this analysis).
The top quarter of those high schools received more than 70 per cent of the high scores awarded. The bottom quarter received less than 2 per cent.
Prosecuting this imbalance reveals a predictable skew towards private and selective schools.
However, there is still a big variation between school performance within the government, non-selective sector.
In fact, looking at the results of only those schools shows the top 25 per cent get an even higher percentage of the high scores - 75 per cent.
Some of it is down to school size, of course, but another highly correlated factor is the level of educational advantage enjoyed by students.
This is measured by the government using an ICSEA score, which takes into account factors such as parents' occupations, education level and the location of the school.
The average ICSEA score is 1000; schools with a higher number are considered relatively advantaged, and those below relatively disadvantaged.
Looking at the 256 non-selective government schools for which ICSEA scores were available reveals a clear relationship between a school's ICSEA score and their HSC success rate (the number of band 6s as a percentage of entries).
Comparing the two results helps reveal schools punching above their weight on educational outcomes.
Forbes High School, for example, is one of the state's 10 most disadvantaged schools, yet has achieved a success rate that would put it in the top 100 of non-selective public schools.
In Sydney, schools such as Canley Vale, Birrong Girls, Fairvale and Chester Hill high schools are performing well above what one might expect.
The following maps look at schools that have performed above expected outcomes based on their ICSEA score.
Asked why some schools do better than others with similar circumstances, NSW Secondary Principals' Council president Chris Presland said it was mostly down to teaching staff.
"The most significant variable between schools is the quality and experience of teachers. If you've got a teacher who's an HSC marker and has been teaching a course for several years, that's a huge advantage to a school," Mr Presland said.
"Some schools also have additional programs and mentoring and a lot of it's got to do with resourcing, but nothing compares to the capacity of the teacher."
However, Mr Presland cautioned against only assessing schools based on their percentage of high scores (band 6s) achieved.
"If you look at what the best school in the state is by band 6s, it's James Ruse every year," he said.
"But if you look at which school has the best teachers and is making the biggest difference, it's never a selective school or private school.
"From a schools perspective, what we use is our value-added data, it shows the growth rate of every student from the year 9 NAPLAN through to the HSC.
"Lower socio-economic schools tend to make a massive difference. That's far more significant than band 6s."