"Streaming" of school pupils into classes of academic ability has long been out of favour, in educational theory.
But in practice it happens, obviously to a far greater extent than schools may want to admit.
Former Auckland University education professor John Hattie tells us in the Herald today streaming may be to blame for New Zealand's poor performance in subjects such as maths and science as measured by the Programme for International Students Assessment (Pisa) tests.
In the latest results our reading scores have dropped by more than all bar three of the 24 developed countries assessed, and our science scores declined by more than all except eight of them. The decline in our maths among pupils aged 15 was the worst since Pisa surveys began in the year 2000.
Hattie says, "We have more streaming than any other country in the world", we also have one of the widest gaps between those who do well in our schools and those who do worst. The Pisa results worldwide suggest countries that stream less do better overall.
The reason is not hard to see. Streaming predetermines children's performance, removing challenges they might have faced in a class of mixed ability, foreclosing the possibility they might be a late improver, permanently lowering, or raising, their confidence in themselves.
No matter how much schools try to disguise streaming by giving classes indeterminate labels, pupils are not fooled. They all quickly work where their class stands by the subjects it is given to study and the assignments they get. Nothing boosts a child's confidence, or lowers it, more than educational comparisons with their peers.
That is the reason school leaving examinations were changed in the 1990s to replace comparative measurements with objective standards. That should have permitted all pupils to work at their own level at their own pace in classes of mixed abilities. But evidently it has not worked out that way.
Just about everybody in education seems to agree streaming is harmful, including the minister, Hekia Parata, and Labour's spokesman, Chris Hipkins, but neither National nor Labour are about to forbid the practice.
It will remain for each school to decide how it places its pupils in various classes.
Professor Hattie says too many schools take the easy course of shielding weaker students from "hard" subjects.
"We are brilliant in New Zealand at getting kids out of science and maths."
Obviously, a move to classes of mixed ability would mean more personalised teaching. There would be no point teachers continuing to pitch the material to the middle of the class if it contains the very bright kids and the slow learners. The bright ones need and deserve a challenge as much as the slow ones need and deserve help.
Parata hopes that her scheme that encourages schools to work together will give pupils more individual attention within classes of mixed ability. Hipkins thinks it will require smaller classes and that will mean more funding.
But the answer may be to stop shielding any of our children from "hard" subjects. Our performance is declining steeply in reading, maths and science. Reading and maths are subjects every citizen needs.
Science is often an acquired interest and a good teacher can awaken a pupil's interest late in their school years. The less we prejudge pupils' abilities the better more of them will do.