Higher decile schools are increasingly reliant on parent fundraising and donations to meet what they say is a shortfall in funds.
Frances Nelson, principal of decile 2 Fairburn School in Otahuhu, said she understood the Ministry of Education would inform schools of their new decile next week.
After the last Census Fairburn was promoted from decile 1, leaving the school with a $70,000 funding shortfall.
"I would guess it's an anxious time for many schools who look around their community and think, 'Do we look more affluent than last time?'" Ms Nelson said.
Decile 1 schools are the 10 per cent of schools with the highest proportion of students from low socio-economic communities, whereas decile 10 schools are the 10 per cent with the lowest proportion of such students.
Mt Roskill's Dominion Road School is decile 3. Some property sales on nearby streets have approached $1 million. Such prices are not a direct factor in decile calculation, but can indicate some residents' income levels, which are.
Board of trustees chairman Paul Brislen said they had not discussed in detail what a new decile could mean. "The area itself has increased in value hugely in the last 10 years, so if it does have an impact we'll have to cross that bridge when we get to it."
Mt Albert Grammar School headmaster Dale Burden said the decile 7 school had budgeted for the "worst-case" scenario of a decile increase.
After the last Census the school went up two deciles. Mr Burden said the system needed an overhaul.
"The whole funding system presupposes that the higher the decile the more likely it is that parents will make the difference up themselves.
"Yet the minute parents are asked to pay anything the ministry tell us it is illegal to charge people anything other than a donation."
John Paul College principal Patrick Walsh, who is on the executive of the Secondary Principals' Association, said there was widespread dissatisfaction with the decile system and he believed a more accurate and precise measure was needed.
Asked if a review was planned, Education Minister Hekia Parata said any change would need to be "well thought through, be well foreshadowed, and would require work alongside the profession and sector groups".
Graham Stoop, the ministry's head of student achievement, said in the last recalculation seven years ago one-third of schools gained a higher decile rating, one-third gained a lower rating, and one-third remained the same.
"We recognise that any reduction in funding will be an adjustment, which is why this time we have introduced an 18-month transition period for any reductions.
"Increases in funding will however take effect much sooner, at the start of the 2015 school year. We will be working closely with schools to give them any support they need to adjust to changes."
