The president of the Wellington Primary Principals Association says schools will be scrambling to explain to parents the context of national standards information published by Fairfax media.
The data for some 2000 schools operating under the standards is to be published on the Ministry of Education's website on Friday.
On Saturday, Fairfax published information on more than 1,000 schools showing the percentage of children above, meeting, below or well below expected standards for reading, writing and maths.
Association president Sandra McCallum, the principal of Mount Cook School in Wellington, says schools will need to reassure their communities about what has been made public.
Ms McCallum says Mount Cook School will send a newsletter to parents saying the data does not reflect the context of the school.
The Principal's Federation says allowing people to compare multiple schools is dangerous, especially on such a narrow range of data.
President Paul Drummond says schools in poor areas, especially small schools, will fare worst with publication of the data.
"Those schools that are in low socio-economic areas, where they are doing remarkable and really incredible things, where there has been significant lifts in achievement now, this will demoralise those schools."
Mr Drummond says the data will ruin the collegial way many schools have been operating within districts, as they seek to compete with one another.
However Education Minister Hekia Parata told Morning Report she's not upset about the publication in Fairfax media.
"NCEA results are published in double spreads of newspapers every year but no-one assumes that is the only measurement.
"Parents go into those schools and they discuss with the teachers and the principals and the board what is happening for their own students, and I encourage them to continue to do that."
Ms Parata says the information should already have been reported to parents when it was first collated at the end of May.