Primary schools in the UK have higher class sizes by OECD standards
Primary school classes in the UK remain "very large" by international standards, says an annual report into education systems.
Primary funding in the UK is above average, says the Organisation for Economic Co-operation and Development.
But it highlights the fact that class sizes in state schools are far larger than in the private sector.
However the OECD analysis says there is no clear link between smaller class sizes and pupil achievement.
The OECD's Education at a Glance 2008 examines the ways that industrialised countries invest in schools and universities.
Among the distinctive features of school systems, the report says "the UK is unusual in having lower class sizes at the lower secondary level than at the primary level".
GDP spending on educational institutions up from 5.2% to 6.2% from 1995-05
Spending above average at primary, below average at secondary
Per pupil spending up 29% from 1995-05
High spending on early years
This report, which does not distinguish between the devolved systems within the UK, says that above-average spending at primary level has not been targeted at class sizes - where there is an average of 24.5 students per class, above the OECD average of 21.5 pupils.
Comparing state and independent schools, it says "the UK stands out as the OECD country with the largest difference in class sizes between public and private institutions", with 13 more pupils per primary classroom in the state sector.
But in presenting the finding, the OECD's analyst Andreas Schleicher says that this could be an efficient use of an education budget.
South Korea, one of the most successful education systems in the world, has even higher class sizes - with an average of 32.
In contrast, secondary school spending is below average - but class sizes are small by international standards, with an average of 22.4 pupils per class.
The report also points to early years education, which has been a priority for investment.
"The UK stands out with respect to its high level of spending per child in pre-primary education," says the OECD - which has created high levels of provision for three to four year olds.
The OECD presents an overall pattern in which industrialised countries are spending an increasing amount on education at all levels, rising by 19% in real terms across the OECD between 2000 and 2005.
The majority of this remains public spending, accounting for 86%, but the OECD notes the shift to private funding, such as tuition fees in higher education, making a more significant contribution as part of the investment.
The Schools Minister in England, Andrew Adonis, pointed to the increased number of teaching assistants in the classroom.
"Over the past 10 years we have seen steadily smaller class sizes and a better adult-pupil ratio thanks to our policies which see 150,000 more adults in classrooms. In 1997 there were 17 pupils per adult at primary level, now there are 12.
"High quality teachers, assisted by dedicated teaching assistants, have made a major contribution to the sharp rise in standards over the past decade."