Average class sizes are generally larger in fee-paying schools than in state schools, according to figures from the Organisation for Economic Co-operation and Development (OECD).

Only in the UK are state school classes significantly larger. We are used to the fact that secondary class sizes in the private sector in Britain are much smaller than in state schools, and indeed are probably among the smallest in the world. This may partly explain why private schooling is now a substantial export industry, attracting pupils from wealthy families around the globe to study in UK private schools, and often to continue at universities in Britain.

One of the reasons private schools have larger classes in most countries may be because those countries are less concerned about meeting parental wishes in the types of schools they will fund. This is especially true of schools run by religious groups.

Faith schools have been funded by the state in Britain since schooling became compulsory in the late 19th century. Indeed, as recently as the late 1990s, the Labour government under Tony Blair extended the funding of such schools beyond the Judaeo-Christian tradition to encompass other faiths. But this is not the case in many other countries, where faith schools tend to be privately run. Faith, rather than quality of education, thus becomes one of the principal reasons for parents to choose those schools for their children.

The other point worth noting is that most developed countries fund secondary schooling to produce average class sizes of 21 to 24 pupils per teacher. The exceptions are Greece, where the average class size in state schools is fewer than 17 pupils per teacher, and Japan and South Korea, where class sizes average well above 30. In these two Asian nations, teachers tend to be well paid and have more preparation time, so there are trade-offs in how the money is spent.

Generally, apart from in the UK, class sizes in private schools are about one to two pupils better or worse than state schools. But few of these countries on the map have such social divisions as there are in the UK. Where state-funded parental choice is becoming more of an option, the line between state and private secondaries may start to become more blurred.

John Howson is director of Education Data Surveys, part of TSL Education.