The new figures show under 11s in the UK are in bigger classes than most other countries, including Poland, Mexico, Slovenia, Estonia and Russia, with the average class size in the UK, 26 pupils. This compares to the average of 21.4 in other countries.
The report put the UK eighth out of 35 countries for primary school class size, and also shows the huge gap between state and private schools in the UK.
The report also shows the gap between state-funded schools and private schools where the class sizes for five to 11-year-olds are half the size.
While the figures for primary school class sizes are pretty bleak, secondary school sizes in the UK shrunk over the last decade to an average of 19.6, below the average of 24 children per class.
The report - Education at a Glance - said UK primary school class sizes had decreased by five per cent over the last decade, but still remain above the OECD average:
'Class size is one factor that parents often consider when deciding on a school for their children and the difference in average class size between public and private schools could influence enrolment,' the study said.
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