PPTA president Angela Roberts said focusing singularly on ethnicity meant overlapping issues like socio-economic status were ignored.
She said the information that 20 per cent of children were failing had created a crisis in schools that was being used to drive through policy change.
"The Government's practice of separating out a single factor - such as ethnicity - and comparing one sub-group to other whole populations is statistically grossly misleading and fails to recognise many factors contributing to under-achievement," Ms Roberts said.
However, PPTA's researchers also found there was around a 20 percentage point gap between the level of achievement of Maori and Pasifika students and non-Maori and Pasifika students in New Zealand.
The research found New Zealand had the highest gap on PISA's social gradient approach - at 52 points. The gap reflected New Zealand's high level of income inequality and a strong performance at the top of the scale.
A spokeswoman for Education Minister Hekia Parata said the PPTA had timed the release of their research with the visit of leading world expert on educational achievement and best practice, Andreas Schleicher this week.
Mr Schleicher is deputy director for education and skills and special advisor on education policy to the OECD's secretary general. He is being hosted by Ms Parata.
Mr Schleicher was not aware of the research and said he could not comment.
The spokeswoman for Ms Parata said 'one in five' was an estimate which reflected the fact that not every person is leaving school with the qualifications and skills they needed to succeed.
"It reflects the fact that 15 per cent of school leavers do not have an NCEA Level 1 qualification and the basic literacy and numeracy skills required to attain it, and that around 30 per cent of students leave school without an NCEA Level 2 qualification - the minimum level of competency required to train for a basic apprenticeship.
"The one out of five reference also drew on ERO research and reading recovery data which indicated that up to one in five young people are leaving school without the skills needed for modern jobs.
"Educationally, the evidence is that students can make good progress based on the quality of teaching they get, not on their socio-economic background. Good quality teaching delivers good outcomes regardless of a child's socio economic status.
"The reality is we have still got a good proportion of young people being underserved by our education system," the spokeswoman said.
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What the research found:
- Average PISA scores in reading, mathematics and science of New Zealand 15-year-olds are high among OECD countries.
- Socioeconomic status seems to affect educational achievement; students with a higher socio-economic status tend of achieve better than those with a lower socioeconomic status.
- New Zealand students perform well on average and their annual gain is higher than the OECD, suggesting education for mid-teen New Zealanders is more successful than a typical OECD country.
- New Zealand's education system is about a year ahead on the achievement measures compared to the OECD average.
