Students in Canberra's public high schools are likely to have the writing and numeracy skills of younger children who have similar advantages in life.
A study of national test results has found systemic underperformance in the ACT, including in private schools, across all socio-economic groups.
The Australian National University research, published on Monday, examines five years of NAPLAN results. It found "an alarming number" of Canberra schools "where the students were, on average, more than six months behind the levels of learning of students in other comparable schools".
The underperformance was most pronounced in public schools but also "a material issue" in the non-government sector.
The worst-performing areas were public high-school students' maths and writing skills. On average, this group's results lagged those of students in similar Australian schools by eight to 12 months' worth of learning.
ACT students' reading results were consistently better than the other two disciplines, though they remained below the results of statistically similar schools in other jurisdictions.
The findings echo those in an Auditor-General's report published late last year. It warned that Canberra's government schools performed more poorly than comparable schools elsewhere "despite expenditure on a per student basis for public schools being one of the highest in the country".
Both reports contrast with the widely held view that ACT schools are among the country's best, because their test results are usually higher than those in other states and territories.
But when factors such as parents' jobs and education levels, wealth of the local area and the number of disadvantaged students enrolled are considered, Canberra results fall well below interstate outcomes.
ANU law professor Andrew Macintosh, who wrote the paper with PhD candidate Debra Wilkinson, said he had tried to encourage the ACT government over several years to study the causes of the underperformance.
"This is a really interesting research question. Why the hell are we seeing these results every year? We are so far behind in writing and numeracy," he told The Canberra Times.
"We thought policymakers would really want to dig into the results and try to fix the problems, if there are any."
The paper poses several potential explanations, such as different teaching practices, different academic expectations, or other jurisdictions placing greater emphasis on coaching for NAPLAN tests.
However, it says it's not possible to reach firm conclusions without further research.
The authors urge the government to set up a public inquiry into the causes of the poor results.
ACT Education Minister Yvette Berry said she could not comment on the findings as the researchers had not provided the government with an advance copy.
However, she said improvements in other states and territories' NAPLAN outcomes had brought them closer to the ACT's historically strong performance.
Why the hell are we seeing these results every year? We are so far behind in writing and numeracy.Professor Andrew Macintosh
"The ACT remains typically the highest, or equal-highest, performer in the vast majority of assessment domains and year levels," she said.
"However, there is more to measuring the quality of school education than performance on a test at a point in time."
Earlier this year, Ms Berry led calls for a national review of the My School website, which publishes schools' test outcomes.
She raised concerns about the way the website is used, particularly how "it facilitates simplistic rankings of schools based on point-in-time testing and puts unfair stress on students, families and teachers".
A review is under way, investigating possible limits on public access to schools' results. It is expected to report later this year.
However, Professor Macintosh and Ms Wilkinson warned against calls to scrap the testing scheme.
"Abandoning NAPLAN would undermine the capacity of teachers, policymakers and researchers to serve students and the broader community in the provision of education services," they wrote.
"Rather than abandoning NAPLAN, we recommend greater efforts be devoted to devising better ways of utilising NAPLAN data to assist teachers and students."
Preliminary literacy and numeracy results for 2018 are scheduled to be released on Tuesday.