Mike Russell, the SNP’s education spokesman, enlisted support from a study by the Institute of Education at the University of London to show that smaller class sizes - even if not precisely his party’s magic ceiling of 18 in P1-P3 - would make a difference to pupil achievement.

No discussion of this issue is complete, of course, without reference to STAR wars between the protagonists - the “student teacher achievement ratio” project, that is, which originated in the American state of Tennessee. Mr Russell enthusiastically embraced its conclusions that “an optimum of 18 or less” in early years classes boosted attainment, sustained it in the later stages of schooling and improved behaviour.

Jack McConnell, Education Minister, did not exactly suggest that size did not matter. But, either because he was embarrassed by the SNP going one better than Labour’s prized pledge of a maximum of 30 in P1-P3 or because he was disdainful of it, his consensual style deserted him. Is there an election on?

Mr McConnell has also scoured the world for research backing, in his case an American review of 277 studies on pupil-teacher ratios, only 15 per cent of which reported improved academic results from smaller classes. In any case, he said, Scottish schools were “well on target” for an adult-pupil ratio of 15:1 next year.

More exotic academic company was kept by Brian Monteith, the Tories’ education spokesman, who called in aid the Kiel Institute of World Economics. Its study, based on 260,000 students in 39 countries, suggested that there could be a more productive way of using resources in bigger classes that would outweigh advantages from smaller sizes.

Uncharacteristically, the iercely partisan Mr Monteith sought a middle path between his opposite numbers. “The jury is out on class sizes,” he said, and “there is no ideal way”.

The battle of the funds was almost as inconclusive as the battle of the academics. Mr Russell said the SNP’s policy would require another 3,115 primary teachers, phased in over seven years. Together with additional expenditure on new buildings and teacher training, the cost would peak at pound;100 million over that period - a mere 0.5 per cent of the Scottish Executive’s budget.

Mr McConnell opted for “a sensible analysis” - 5,000 extra teachers would be needed, at a cost of pound;168 million a year excluding training and professional development, with another pound;350 million to build 5,000 extra classrooms and pound;23 million a year for their upkeep. The policy would benefit fewer than 10 per cent of Scottish primaries, he said.

One of the schools that would not benefit would be the Royal High primary in Edinburgh, Mr Monteith disclosed. His sons’ former school would have to lose six pupils from each of the first three primary years and introduce three classrooms with an additional three teachers. Or, as the politician who is keenest to portray himself as the parents’ champion put it, “that would mean a possible 36 more disappointed parents”.

Mr Monteith has been playing with his calculator too and reckoned the SNP’s policy would require not Mr Russell’s 3,115 extra teachers or Mr McConnell’s additional 5,000 but a minimum of 2,542 new teachers.

Where would they all come from, Mr Monteith cried, given the potential problems of recruiting another 4,000 teachers as a result of the teachers’ settlement? No problem, Mr Russell said, since there are nine applications for every primary training place.

Altogether a bewildering morning.

Leader, page 16