An official report has cast serious doubt on last year's results for the national standards in reading, writing and maths.
The independent study commissioned by the Ministry of Education says teachers' use of the standards last year lacked dependability.
The report is based on research involving 96 schools and hundreds of teachers.
It found teachers' hit rate for correctly ranking children's work against the standards was about 60%.
The study found big differences in the pass rates of intermediate and primary schools and says the proportion of poorly performing children who improved last year seems unreasonably large.
The report says the evidence suggests teachers' judgements lack dependability and there is no way of knowing how many of last year's national standards results were correct.
The New Zealand Principals' Federation says the report shows the assessment system is shonky and unreliable.
Federation president Phil Harding says the findings show it's very hard for teachers to make accurate Overall Teacher Judgements using a fundamentally flawed system.
"The report comes as a complete slap in the face for those positions. We have been saying the data is not valid and reliable for the last four years, so it is a great vindication to have that put in black and white and to realise that if the OTJs are wrong so are all the deductions that flow from that." he said.
Education Minister Hekia Parata says she expects the accuracy of the results to improve.
She told Morning Report it is only the second year results have been reported and the ministry has a five year plan.
Ms Parata says national standards do not replace parents talking to their child's school and teacher.