An educator pushing for the end of streaming in schools says while putting kids in classes with "does seem a little bit intuitive", it results in worse educational outcomes overall.
And those most put at a disadvantage are Māori, Pasifika and children from lower-socioeconomic backgrounds.
"Kids who are put in the lower levels are taught lower-level things, and have slower progress," Dr David Pomeroy, senior lecturer in teacher education at the University of Canterbury and an ex-secondary school teacher, told RNZ on Monday.
More than 80 percent of schools in New Zealand use streaming in some form - the third-highest rate in the OECD. Last year both the Post Primary Teachers' Association and its primary school equivalent, NZEI Te Riu Roa, said they wanted it gone.
In an April 2021 briefing to then-Minister of Education Chris Hipkins and his successor Jan Tinetti, the Ministry of Education said the "evidence shows that streaming contributes to inequitable outcomes, especially for Māori learners, Pacific learners and learners with disabilities and/or learning support needs".
"Kids who are put in the lower levels are taught lower-level things" - Dr David Pomeroy, senior lecturer in teacher education
Supporters of streaming say putting children of differing abilities in the same classroom holds back those of exceptional ability.
"It kind of makes sense, but when you talk to the children who are in these groups, you get a really different story," Pomeroy said.
Kids placed in lower streams got further and further behind, he said, their educational potential set in stone as young as age six.
Māori, Pasifika and children from lower socioeconomic backgrounds were more likely to be placed in lower-ability streams regardless of their potential, the research showed.
"When you put students into groups based on their prior achievement, we're really saying, 'We know what your potential is, we know what you could achieve in the future.'"
A new Māori-led plan to end streaming by 2030, 'Kōkirihia', was launched on Monday. Involved were NZEI, PPTA, the Ministry of Education, iwi leaders, NZQA, the Education Review Office and other groups.
"It is a big shift for any school to make, but it's the right thing to do," NZEI president Mark Potter said. "As educators, we are used to changing what we do to reflect best practice."
Pomeroy said the evidence showed there was little benefit for high-achieving students in streaming, and some little-acknowledged downsides.
"There's extensive evidence internationally and within New Zealand that the low streams are harmful to students in those streams; what isn't really intuitive but what research has shown again and again is there's really not much of a benefit being in a top group - you don't see improvements to achievement.
"That fear that somehow the highest achievers are being sacrificed to bring up the low achievers just isn't substantiated.
"You also tend to see a lot of anxiety, a lot of competitiveness. There's a few kids who thrive on that… but for a lot of children, that's not helpful."
And kids placed in lower-ability classes are more likely to give up on learning altogether, their confidence shattered.
Pomeroy said schools which had moved away from streaming, such as Horowhenua College, were reaping the benefits of mixed-ability classrooms.
"What comes through really clearly is there's often a lot of trepidation and uncertainty amongst teachers who are taking that step away from streaming. But often they are surprised by the results, and they're really surprised by what students who they would previously have perceived as not a top student, not academic, who would have been assigned to lower-level class, what they can be capable of…
"The students coming through haven't changed, but they've stopped putting them in groups that place a cap on their achievement."
But across-the-board change would not be easy, even with teachers on board, Pomeroy said - which was why the Kōkirihia campaign was aiming for 2030, not tomorrow.
"Teachers are rightly saying, 'Sure, I can see the why for this, but give me the how'. And there is some professional development and some adaptions that are needed to teach a wider range of students in your class than you've ever had before.
"So sure, you can end streaming overnight… but to make the most out of that, teachers need to have a few new tools, and they need the resourcing and the time to adapt."