Tens of thousands of middle-class parents from around the world are paying thousands of pounds a year to send their teenage children abroad for an education at private schools in English-speaking countries.

Each nation's moneyed class has a different motivation, say head teachers and admissions tutors in the UK, US and New Zealand. Fees can be as much as £20,000 ($39,460, €26,510).

South Korean parents top the list for their levels of ambition, say many schools. Michael Gary, head of admissions at the co-educational Phillips Academy, Exeter, New Hampshire, one of the top-performing US schools in terms of Ivy League admissions, concludes: "They're very serious about their education. Their mindset is: 'Exeter, Harvard or bust'."

Chinese parents value academic rigour highly, with many UK-educated children going on to Britain's top-tier "Russell Group" universities to study science and maths. But they also want to immerse their children in a "less deferential and didactic" western education, says Philip Evans, headmaster at Bedford School for boys in Britain, whose intake is 14 per cent from overseas.

Some German parents "seem increasingly despairing of the German system, which offers large classes, surprisingly weak discipline and little extracurricular activity", says Mark Eagers, headmaster at the co-educational Box Hill School, where foreign pupils fill about a quarter of the class.

But what foreign parents have in common is an eagerness to see their children educated with Britons rather than many other foreigners.

Harrow School, a boarding school for boys in London, "could fill itself entirely with overseas applicants", says Barnaby Lenon, the headmaster. However, it "informally" sets a limit to the proportion of foreigners at "about 12 per cent", primarily "because overseas parents themselves do not want a large number of boys of their own nationality". From a practical point of view, Mr Lenon says, parents want their boys to learn English well, which is more likely if they are not associating with their compatriots. Less tangibly, heads say children have come to British private schools to enjoy an unalloyed "British" education.

Many British schools cannot afford the same scruples. Some rely on overseas pupils for a third or more of their intake. One head says that unless the situation is well managed, it creates "ghettos" of pupils who make slower progress at English.

In the worst cases, he says foreign business is propping up schools that do not provide a good education.

Parents afraid of sending their children away to a far-off land can instead opt for enrolling them in their own country, at one of a multiplying number of academies linked to British schools, usually through a franchise. There is, for example, a Harrow International Bangkok and a Harrow International Beijing, in a franchise owned by Daniel Chiu, a Chinese businessman. The school demands safeguards, says Mr Lenon, to ensure the quality of education is equal to that provided in London.