Millions of dollars in government funding are at stake as the Ministry of Education gets ready to recalculate schools' decile numbers using the latest census data.
The numbers, from 1 to 10, are based on the proportion of children at each school from the most disadvantaged homes, and are used to allocate extra funding where it is needed most.
A change in decile can be worth tens or even hundreds of thousands of dollars, and in 2007 when the numbers were last revised, two-thirds of schools were reclassified.
On 23 July the ministry begins ensuring it has addresses for all schoolchildren so it can recalculate the deciles.
Secondary Principals Association president Tom Parsons said many principals fear their decile will rise, causing their funding to drop.
"Resourcing is the biggest issue fronting, I would argue, all principals but certainly secondary principals today so any shifting on that funding base for schools, principals do watch ... very, very carefully."
Principals expect population movement will mean a lot of changes for Christchurch and Auckland schools.
The ministry says schools that qualify for more funding will get it from January, but cuts will be applied in stages.