The head of the Principals' Federation is dismissing the idea bigger class sizes can mean a better education.
Detailed National Standards data for specific schools is now in the public domain, after the Education Minister released limited figures earlier this week.
One survey shows classes with a high student to teacher ratio outperforming others with lower ratios.
But Principals' Federation President Paul Drummond thinks it's nonsense.
"I don't think there'd be any really strong evidence around that correlation and I think that would have to be tested a little bit further. I really think that might be a little bit ambitious to be making any strong conclusions or claims from that."
He is also raising doubts about the benefit of making National Standards data public.
The Education Minister released some information this week - listing averaged scores for primary school children's reading, writing and maths.
But weekend papers have compiled their own tables, giving a lot more detail on how well different schools are doing.
Paul Drummond says there are major risks on making the information public.
"It can't possibly reflect the performance or achievement of a New Zealand child or its school, and as it's been proven overseas and indeed in New Zealand over a hundred years ago, it has the potential to distort our rich curriculum."