Education leaders and academics have challenged the Government's figures on the number of children failing at school.
They say the failure rate is lower than the 20% cited by the Minister of Education, Anne Tolley.
Radio New Zealand's education correspondent says Ms Tolley repeatedly states one in five school children is failing, using the figure a reason for the Government's national standards in reading, writing and maths.
Some in the education sector agree with her, but others say the failure rate is one in seven or 15%.
That figure is supported by the NCEA level 1 pass rate, testing of primary school children, and international studies of reading and maths.
In 2008, only 15% of school leavers did not have at NCEA level 1, though 29% failed level 2.
Lester Flockton, a former director of the National Education Monitoring Project which regularly tests primary and intermediate school children, says those tests indicate 10-15% of students struggle and show no pattern of decline in achievement over the years.
The Council for Educational Research creates school tests including the Progressive Achievement Tests (PAT).
Senior researcher Charles Darr says the proportion of children failing depends entirely on the benchmark used to measure failure, and no matter what is measured, there will always be a bottom 20%.
The minister says her figure is based on ERO reports, leaving statistics and and the NCEA level 1 failure rate, and the higher level 2 failure rate.
"When you put all those together you get just under one in five," Anne Tolley says.
But the president of the Educational Institute, Frances Nelson, says the minister might be using the higher figure to garner support for national standards.
Ms Nelson says even if that is not the case, the higher figure applies to secondary schools and should not be used to justify national standards for primary schools.