For many years, conventional wisdom has stressed the importance of reduced class sizes. The outcome of this was indicated yesterday by the Education Minister, Hekia Parata, who said that at her primary school, there had been as many as 42 pupils for every teacher.

There was good reason to improve that ratio. Struggling pupils, in particular, gain from the greater opportunity for one-on-one tuition afforded by smaller numbers in a classroom. The minister was, therefore, inviting a strong backlash when she signalled that class sizes are to increase.

In an ideal world, this would not happen. Even the Treasury, which suggested the move this year, believes class size does matter. But it said the quality of teaching mattered more, and in a world where Governments had to make trade-offs, this was one that would have minimal effect on pupil achievement.

Using much the same language, Ms Parata announced that a standardised teacher-pupil ratio in Year 2 to 10 classes would free $43 million each year over the next four years to improve teacher quality. "We are opting for quality, not quantity, better teaching, not more teachers," she said.

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This policy is based on new research, led by an Australian think-tank, the Grattan Institute, which suggests improving teacher quality is far more cost-effective than reducing class size. To that end, the Government will invest an extra $60 million over four years to boost teacher recruitment and training. A post-graduate qualification will become the minimum requirement for all trainee teachers, and a new teacher "appraisal system" will be developed. Ms Parata said performance pay was one of "a basket of options" to recognise and reward teacher quality. It should, in fact, be at the head of any moves to encourage excellence in the classroom.

There can be few qualms about the accent on quality. The Treasury has suggested the effect on pupil learning of moving from a class with an average teacher to one with a high-performing teacher is roughly equivalent to the effect of a 10-pupil decrease in class size. That calculation points also to the importance of class size. Any increase in the teacher-pupil ratio must not be the start of a slippery slope.

Ms Parata says the new ratio would mean 90 per cent of schools would gain or have a net loss of less than one fulltime equivalent teacher. If so, there will surely be little difference, especially if there is an improvement in teaching.

Not that this will placate the teacher unions. The New Zealand Educational Institute warned immediately of a "severe impact", especially on the 20 per cent underachieving tail. It claimed, also, that this was a case of the Government taking advice from the Treasury and not listening to the education sector.

That is no bad thing. The Treasury recommends policies without taking account of the political risk in implementing them. For that reason, its advice is rarely heeded. All too often, however, that is not a reflection of the quality of its advocacy. Its post-election briefing paper, for example, commendably recommended increasing the retirement age from 65 to 67, as well as bigger class sizes as a way of finding money to improve teacher effectiveness.

Research indicates that a Californian programme of class size reduction faltered because the need for more teachers led to a deterioration in their qualifications. This offset the gains from smaller classes.

The same may not have happened in New Zealand, but parents, especially, will welcome the focus on quality. If asked to choose between their children being in a slightly smaller class or having an excellent teacher, most would surely opt for excellence.