Results from the prestigious PISA tests of reading, maths and science are due out in December but we already know New Zealand's scores are likely to be 10 points higher than they should be.
So many schools refused to participate in the OECD's Programme for International Student Assessment last year that the sample of 15-year-olds was not fully representative.
The Education Ministry says there were more students from private schools and high-decile schools and more students who went to classes regularly.
Education correspondent John Gerritsen has been looking at the ministry's report.
