Media playback is unsupported on your device Media caption Sean Coughlan examines the OECD's global education rankings

The biggest ever global school rankings have been published, with Asian countries in the top five places and African countries at the bottom.

Singapore heads the table, followed by Hong Kong, with Ghana at the bottom.

The UK is in 20th place, among higher achieving European countries, with the US in 28th.

The OECD economic think tank says the comparisons - based on test scores in 76 countries - show the link between education and economic growth.

"This is the first time we have a truly global scale of the quality of education," said the OECD's education director, Andreas Schleicher.

"The idea is to give more countries, rich and poor, access to comparing themselves against the world's education leaders, to discover their relative strengths and weaknesses, and to see what the long-term economic gains from improved quality in schooling could be for them," he said.

The top performer, Singapore, had high levels of illiteracy into the 1960s, said Mr Schleicher, showing how much progress could be made.

In the UK, the study shows about one in five youngsters leave school without reaching a basic level of education - and the OECD says that reducing this number and improving skills could add trillions of dollars to the UK economy.

"I think it's partly a mindset, an expectation. There are plenty of examples of schools that have raised the bar dramatically," said education minister Lord Nash.

But a leading UK head teacher, Sir Anthony Seldon, criticised such league tables as "arguably doing more harm than good".

"They are skewing schools and national education systems away from real learning towards repetitive rote learning," said Sir Anthony, head of Wellington College in Berkshire.

The analysis, based on test scores in maths and science, is a much wider global map of education standards than the OECD's Pisa tests, which focus on more affluent industrialised countries.

Image copyright Reuters Image caption Hi-tech vision of the future: Experimenting with technology in Singapore

This latest league table, ranking more than a third of the world's nations, shows how countries such as Iran, South Africa, Peru and Thailand would appear on an international scale.

It shows once again the poor performance of the United States, slipping behind successful European countries and being overtaken by Vietnam. It also highlights the decline of Sweden, with the OECD warning last week that it had serious problems in its education system.

Figures mapped above show estimated growth in GDP over the lifetime of pupils. The figures assume that all pupils are enrolled in schools and that they achieve at least basic skills.

More stories from the BBC's Knowledge economy series looking at education from a global perspective and how to get in touch

The rankings are based on an amalgamation of international assessments, including the OECD's Pisa tests, the TIMSS tests run by US-based academics and TERCE tests in Latin America, putting developed and developing countries on a single scale.

The findings will be formally presented at the World Education Forum in South Korea next week, where the United Nations is to convene a conference on targets for raising global education by 2030.

'Every student to succeed'

The top five places are all taken by Asian countries - Singapore, Hong Kong, South Korea, Taiwan and Japan.

The five lowest-ranked countries are Oman in 72nd, Morocco, Honduras, South Africa and Ghana in last spot.

"If you go to an Asian classroom you'll find teachers who expect every student to succeed. There's a lot of rigour, a lot of focus and coherence," says Mr Schleicher.

Image copyright Reuters Image caption Vietnam has overtaken the US: An Italian scooter company's factory near Hanoi

"These countries are also very good at attracting the most talented teachers in the most challenging classrooms, so that every student has access to excellent teachers."

The report, published by the OECD and written by Eric Hanushek from Stanford University and Ludger Woessmann from Munich University, argues that the standard of education is a "powerful predictor of the wealth that countries will produce in the long run".

Take the test: How would your maths skills compare with the world's teenagers?

"Poor education policies and practices leave many countries in what amounts to a permanent state of economic recession," says the report.

Millennium targets

Improving education would produce "long-term economic gains that are going to be phenomenal", says Mr Schleicher.

If Ghana, the lowest ranked country, achieved basic skills for all its 15-year-olds, the report says that it would expand its current GDP by 38 times, over the lifetime of today's youngsters.

Only a minority of countries in Africa had sufficient test data to be included in these rankings - and it could be that countries such as Ghana are among the higher achievers in the continent, performing better than the majority for which there are no comparable figures.

Image caption Education standards are a "powerful predictor of the wealth" says the OECD's report

The report will provide evidence for next week's World Education Forum of how achieving education targets can deliver economic gains.

The milestone conference, under the auspices of the United Nations, will mark 15 years since the setting of education targets by world leaders.

These millennium targets for education, such as providing all children with a primary education, have not been fully achieved.

But the World Education Forum will set another round of global targets for the next 15 years.