Higher pay for teachers may help primary pupils' results more than reducing class sizes, according to the Organisation for Economic Co-operation and Development.
Presenting the OECD's Education at a Glance report on Tuesday, Andreas Schleicher said that given that most countries could not increase spending per pupil "it may be better to pay teachers more and raise class sizes".
Despite spending more than 20 per cent per primary pupil above the average OECD level - the UK, with 26 children per class, has among the largest class sizes in the OECD. Only Turkey (27 pupils), Japan (29) and Korea (35) have larger, and in all but nine OECD countries there are between 16 and 21 pupils per class.
However, with primary teacher salaries in the UK higher than the OECD average, Mr Schleicher said the UK may have opted for "a more sensible spending choice". Greece and Italy both have smaller average class sizes but do not have such good results. "There may be an overemphasis on class sizes," Mr Schleicher said.
Korea and Japan, with large primary class sizes and high teacher salaries, have shown that large class sizes are not detrimental to young children's education, he said. In Italy a declining birth rate has kept teacher salaries low, while Athens has a policy to keep classes small. Spending per pupil is relatively high but standards are not correspondingly higher than other OECD countries.
Mr Schleicher insisted he was not saying that class size does not matter.
"If a country is willing to make the investment, smaller class sizes do make a difference."
"Studies have shown that larger classes with higher qualified teachers do better than smaller classes with less qualified teachers," Mr Schleicher said. "Evidence also suggests that teachers may not always use the time with small classes to give children individual attention," he said.
Nordic countries and Finland spend much more than the OECD average on primary education - almost 75 per cent above the OECD average per pupil - and maintain both small class sizes and highly paid teachers. "Most countries must choose between the two," Mr Schleicher said.