A replacement for the school decile system has been designed to lift outcomes across the education system.
Thinktank the New Zealand Initiative looked at how three secondary schools performed over 10 years, relative to themselves and every other high school across the country.
The resulting report, Insights and Excellence: School success in New Zealand, uses only existing data drawn from the Ministry of Education and other Government agencies.
Policy analyst Joel Hernandez, who authored the report, says it looks at how schools are really faring.
"It's an objective, external and fair measure that principals and boards of trustees can use to figure out, what are the areas that we're doing really well in, and what are the areas that we need to improve on."
The report separates "the contribution of each student’s family socioeconomic background from the contribution of each school".
"This new report isn’t about giving gold stars to New Zealand's high performing schools or creating new league tables," Hernandez says.
"We want to demonstrate that the Ministry of Education now has the opportunity to identify the high-performing schools - regardless of decile - and learn about the most effective practices for getting the best education outcomes for every student."
The Government is planning to drop the decile system and replace it with something called the equity index, which the Ministry of Education says "better reflects the actual level of socio-economic disadvantage faced by children than the school decile system".
Rather than base funding on the location of the school, the equity index will be measured on the socio-economic disadvantages faced by the school's actual population.
"Unfortunately this misses out on providing parents with information on how our schools are performing," said Hernandez.
The report says that without objective data, "parents are forced to rely on anecdotal evidence and other unreliable proxies to inform their decisions".
"For the past two decades, school quality has been inferred from decile ratings and league tables. Together, these highly flawed proxies for school quality have fuelled decile drift and socioeconomic segregation in New Zealand secondary schools."
Since deciles were introduced in the 1990s, the number of kids attending high-decile schools has risen, while the number at low-decile schools has dropped.
"Even when low-decile schools are marked in the high-performing category... some families bypass them and enrol their children in average-performing high-decile schools outside their school zone."
Hernandez says it's a myth that decile is representative of school quality.
"One decile one school that we have performs in the top 25 percent of all schools in the country on University Entrance."
The full report can be read on the New Zealand Initiative's website.
