The Government says the number of students in low-decile schools will rise by 8000 next year - the first significant increase in more than a decade and the biggest in nearly 20 years.
The jump is due to the recalculation of school deciles based on the 2013 census.
Since 2002, the number of children in decile 1 to 5 schools has increased only one other time - by 150 students this year. Next year's increase is much larger and will be the biggest since 1997.
Based on current Ministry of Education figures, the change will take the number of children in decile 1 to 5 schools to about 311,000 compared with about 450,000 in decile 6 to 10.
The deciles are based on the proportion of children at each school who come from the most disadvantaged neighbourhoods based on measures including household income, overcrowding and qualifications.
They have been recalculated based on census information, and the Government says 800 schools are moving down one or more deciles, while 784 schools are going up.
Children in lower decile schools attract more funding than those in higher deciles and the Government says the increase in students at lower decile schools will cost an extra $6 million a year.
It says schools with lower deciles in 2015 will get their increased funding immediately, but schools with higher deciles will move to their new, lower funding rates in stages.
The lowest decile schools get up to $1000 per student, while decile 2 schools get about half that amount and decile 10 schools nothing at all.
School leaders and Education Minister Hekia Parata have recently questioned the value of the decile funding system and several groups, including the PPTA, say it should be reviewed.
Today she said she did not know why the recalculation of deciles had resulted in the increase in the number of children in low-decile schools, and the Government was looking into it.
"We're looking at the demographic shift, how much of it is related to regional economies, driving parents to those areas, how much of it is that schools are using decile rankings as either stigmas or brands," she said.
"There are a range of reasons why that might be the case and we're interested to understand that."
Ministry figures show 40 percent of school students attend schools in deciles 1 to 5 , compared with 48 percent in 1996.
Radio New Zealand has previously revealed that the number and proportion of New Zealand European students at schools in deciles one to three has dropped dramatically in the past 12 years.