Over two years, ACT students are at least two to three months behind comparable students nationally in numeracy, reading and writing, according to Measuring student progress: A state-by-state report card, published by the independent Grattan Institute.

The researchers take into account factors like how privileged an area is - by how many highly educated parents live there - and then make adjustments to factor this privilege out in their assessment of performance.

Once that's done, the authors conclude: "The ACT is the worst performer. On a like-for-like basis, its students make two to three months less progress (in two years) than the national average in both primary and secondary school."

And that ACT underperformance, the researchers conclude, is getting worse.

Eight years ago, ACT students were making as much progress as those on the national average in reading. They "made around two months less progress than the national average in numeracy".

But four years later, ACT students "made five months less progress than the national average in numeracy, and four months less in reading".

While the researchers are unambiguous that the ACT is bottom of the list, they said Queensland "is the star performer in primary school" and "NSW is great at stretching advantaged students in secondary school" while schools in disadvantaged parts of Victoria make a lot of progress.

Contrary to a common view, Tasmanian schools do well when you take into account how disadvantaged they are. And slow progress by students in country schools is "mainly explained by high levels of disadvantaged students". Nor does size matter: "Whether a student goes to a big or small school has little relationship to how well they will learn."

The ACT's underperformance is worst when it comes to numeracy.

In primary school and secondary school, students in Canberra make by far the least progress compared with other states and territories - more than three months less progress than the national average.

The institute's researchers, Julie Sonnemann and Peter Goss, wrote: "ACT students are, on average, more socio-economically advantaged than students from any other state or territory. Once this relative advantage is taken into account, the ACT trails the nation".

The report doesn't identify with complete certainty any one cause for the ACT's poor performance but one of the authors, Julie Sonneman, told Fairfax Media that it may be because its teachers don't assess accurately how much progress students are making - making it hard for them to pitch lessons to the best advantage.

The researcher thought that teachers in the ACT might not be using all the available "assessment data".

She also questioned whether teachers in the ACT were as "open to change" as teachers in other parts.

The Grattan Institute report echoes some of the findings made by the then auditor-general last year. Dr Maxine Cooper concluded that the territory's schools performed worse than similar schools in other states despite the ACT's funding per student being among the highest in the country.

Dr Cooper questioned whether the autonomy given to ACT schools meant that schools were using too many different ways of assessing their students. "For a small jurisdiction such as the ACT this is excessive", she said.