The Prime Minister's favourite education guru, John Hattie, a former University of Auckland professor now based in Australia, can put a precise value on how big an effect class size has on achievement, compared with, say, "quality".
Hattie's analyses of hundreds of education studies, outlined in his influential book Visible Learning: A Synthesis of Over 800 Meta-Analyses Relating to Achievement, gives "Quality of Teaching" an average size effect of 0.44 and "Teacher-Student Relationships" an even better 0.72. The effect size of smaller class sizes is only 0.21 - a smallish plus.
As Hattie himself cautions, "The reader is reminded that meta-analysis is a method of literature review - the lack of effects from lowering class size summarises the experiences of past reductions in class size and these experiences indicate that reducing class sizes has not been a powerful moderator on outcomes (although the positive sign of the average effect size suggests that increasing class size is poor policy)."
The Government argues, though, that faced with the kind of hard choices that, frankly, whingers in the cheap seats should be glad they don't have to make, it has sensibly chosen to direct more money towards the clearly more effective "quality teaching".
Bigger classes, it says, will deliver quality education and raise achievement.
It's all about fiscal prudence and evidence-based policy. Quality over quantity, the embattled Education Minister Hekia Parata keeps grimly reciting.
The fact that apparently scarce funds are also being directed into charter schools, which by Hattie's measure are even less effective than class size (a .20 effect size), is not at all inconsistent.
No more inconsistent, anyway, than safeguarding smaller classes for Maori language immersion schools, or sending one's children to private schools for the advantage of their smaller classes, or requiring teachers to gain a post-graduate degree while pulling the financial rug out from under them.
Out in the real world, where "average size effects" mean nothing, there are kids whose disadvantaged beginnings aren't being countered by our world-class education system. And they aren't being helped while hostilities continue between the Government and the education sector.
Last month, an Education Review Office report criticised the lack of progress for Pacific students, "the learners most at risk of not achieving in New Zealand schools".
No one knows better than Ant Backhouse, the "white dad" to 31 low-income mostly Pacific Island high school kids, how difficult it is to raise the achievement of under-privileged children. But some schools, he says, are entrenching that disadvantage by putting kids on "a pathway to nowhere".
Backhouse is ideally placed to understand why low-income Maori and Pacific kids fail. And he thinks systemic inequalities help explain "the discrepancy between our students' achievement and those of their peers".
It's been 10 years since he became the full-time co-ordinator for the "I Have A Dream" project, taking on a class of 8-year-olds in a low-decile Mt Roskill school.
Based on an American model, its aim was to improve the odds of disadvantaged kids making it to university by giving them the support middle-class kids take for granted.
Its very hands-on sponsor is Auckland businessman Scott Gilmour, who promised his "Dreamers" he'd pay their way through university if they stayed at school and worked hard.
But he didn't just leave them to get on with it. The Dreamers were given a place to study, fully equipped with computers, an army of volunteer tutors and mentors, field trips to broaden horizons narrowed by poverty, and an advocate in Backhouse who did what middle-class parents do for their kids: hold their hands and fight for them when the occasion demanded it.
The Dreamers are now in their last year of high school.
It's something of a minor miracle that, of the 31 Dreamers still remaining (from 53 originally; the others moved, including 15 to Australia), 10 are placed to sit NCEA Level 3 and 12 are poised to gain University Entrance.
That may not seem like much but in a comparison group from the same primary school, 20 students finished Year 13, but only one got university entrance and one other achieved NCEA Level 3.
For the other 18, the pathway to university had been closed off years earlier. Why? Because as early as Year 9, as an IHAD discussion paper points out, kids are being "locked into a pathway that cannot lead to academic achievement or university".
This is because some schools limit access to subjects that are necessary if a student is to advance to higher education. Some are put into streams that do only "soft" unit-standard subjects that neither they nor their parents realise won't lead to a qualification until it's too late.
Backhouse believes the league-table mentality is partly to blame; some schools don't want to tarnish their impressive-looking achievement stats by letting kids who they think will fail try harder academic subjects.
"It's not fair, and it's a nonsense if we are serious about preparing students for higher education and/or meaningful careers," he says.
There must be "a fundamental assumption and hope that they can succeed".
Backhouse doesn't underestimate the enormity of the challenges. "We realise many of the problems students bring to high school started in early childhood."
But that can't be allowed to be the end of it.
The Dreamers show that Maori and Pacific students, "placed and supported on the right path ... will achieve academic success".
Recognising how much support disadvantaged kids need is a necessary first step to making a difference for them. But nothing will change without a genuinely collaborative approach.