Small classes are a strong selling point for independent schools. The Headmasters' and Headmistresses' Conference (HMC) representing heads of independent schools, mostly in England, contrasts the vastly different pupil-teacher ratios in the independent and state sectors. It claims that its 9:1 ratio is "proven to improve academic achievement".

But does it? To most observers, there must be a correlation between fewer pupils per teacher and improved learning and achievement. That thinking possibly lay behind the Scottish Government's commitment to reduce class sizes, particularly at the early stages in primary schools. However, the average primary class increased in size from 22.7 pupils in 2012 to 23.2 in 2013. The percentage of primary one to three pupils in classes of 18 or fewer fell from 18.8 per cent to 13.6 per cent over the same period.

Cue gnashing of teeth by opposition parties and teachers' unions. The Government's commitments on education were being broken, wailed the Educational Institute of Scotland. Labour weighed in with talk of abject failure. Surprisingly, it was the Conservatives who probably got it right by suggesting the Government's class size commitments had been neither desirable nor deliverable.

The outcome of research into the impact of class size on learning and achievement is inconclusive at best. Evidence suggests that the youngest children do better in smaller classes. But there is less evidence to demonstrate that older children, particularly in secondary, achieve more in smaller classes. The HMC is probably correct but only in the context of class sizes unachievable within the state sector.

Professor Peter Mortimore, highly regarded director of the Institute of Education in London, has warned of the law of unintended consequences: "One worry might be that reduced class sizes could make teachers' lives easier without necessarily improving the quality of the curriculum," he said.

Uncertainty about the effectiveness of reducing class sizes also emerged from research sponsored by GEMS education solutions. The research produced an "efficiency index" for 30 industrialised countries calculated using a ratio of education spending against results arising from the Programme for International Student Assessment. Finland and South Korea occupied the top spots, the UK was 11th. Surprisingly, two of the highest spending countries, Germany and Switzerland, were in the bottom five.

Class sizes in Britain are significantly higher than in most Organisation for Economic Co-operation and Development (OECD) countries. But, as the OECD has pointed out, although spending on education has increased by 30 per cent over the past 10 years, learning outcomes have remained flat. Investment to reduce class sizes and increase teachers' pay has not automatically produced better results.

Many feel investment to marginally reduce class sizes could be better spent, for example by restoring lost classroom assistant posts. This could also further develop teachers' professional skills. A poor teacher with a big class will probably still be a poor teacher with a small class.