Poor children in New Zealand are more than six times more likely to do badly at maths than well-off children, a new report from the OECD says.
The report, "Low-Performing Students - Why They Fall Behind and How To Help Them Succeed", found the disadvantage of poverty was made even worse by factors including a lack of early childhood education or coming from a single-parent family.
It was based on the results of tests of 15-year-olds conducted in 2012 for the Programme for International Student Assessment (PISA).
Initial reporting in 2013 said New Zealand's results were above average, but had worsened and socio-economic background accounted for 18 percent of the difference between New Zealand children's results in mathematics.
The new report looked at the results differently. It concluded that the poorest 25 percent of New Zealand students were more than six times more likely to do badly in maths than those from the richest 25 percent.
That was after taking account of other factors such as coming from a single-parent family or from an immigrant household, and was higher than the OECD average of four times.
The only OECD member nations with higher rates were Israel, Poland and Ireland.
'Most important risk factor'
The report said socio-economic status was "probably the most important risk factor associated with low academic performance" around the world.
It said other factors also affected students' performance. For example, New Zealand students from a single-parent family or from non-English speaking homes were about twice as likely to be low performers as other students.
Other important factors included not attending early childhood education and whether the child had repeated a year level.
However, the report said the other factors associated with poor performance had a greater effect on children from poorer families.
"Among low performers the combination of risk factors is more detrimental to disadvantaged than to advantaged students.
"Indeed, all of the demographic characteristics considered in the report, as well as the lack of pre-primary education, increase the probability of low performance by a larger margin among disadvantaged than among advantaged students, on average across OECD countries."
The report said poor performers tended to do less homework, have less perseverance and motivation, and were more likely to skip classes than higher achievers.
Thirty-five percent of New Zealand's low-performers in the 2012 PISA tests had skipped school at least once in the two weeks prior to the tests, compared to about 12 percent of those who did better in the tests.
Though New Zealand had stronger than average links between socio-economic status and low achievement, it did not have more low-achievers than most other developed countries.
The percentage of students regarded as "low-performing" in maths (22.6 percent), reading (16.3 percent) and science (16.3 percent) were all slightly below the OECD average, as was the 11.1 percent figure for students who were low performers in all three subjects.
The report recommended OECD nations target poor performance, ensure schools were supportive but also demanding, and provide under-achievers with remedial teaching as soon as possible.
Post Primary Teachers Association president Angela Roberts said ensuring rich and poor students went to the same schools would help lift achievement.
Ms Roberts said the OECD's analysis shows results were better in countries where schools have a mix of students. Lower performers benefitted without undermining the high performers, she said.
Schools in poor neighbourhoods also need a lot more resources to improve student achievement.
The most recent PISA test was conducted in 2015 and results were expected to be published late in 2016.