Pupils at independent schools do better because they are surrounded by other privileged children, not because of the schools themselves, the Organisation for Economic Co-operation and Development report found.
Schools with a high concentration of pupils from well-off families benefit from a "peer effect" which increases the advantages of already privileged children. They also enjoy improved discipline and find it easier to attract good staff, according to the second Programme for International Student Assessment.
"Private schools may realise a significant part of their advantage not only from the socio-economic advantage that students bring with them, but even more so because their combined socio-economic intake allows them to create an environment more conducive to learning," the report said.
It also suggests that UK private schools are among the most elitist in the world.
The Pisa study examines the performance of 250,000 15-year-olds in 41 countries in reading, maths and science tests.
Countries with selective systems were more likely to have wide differences between the attainment of pupils, it found. To improve attainment in countries such as the UK, where social class is closely linked to underperformance, policies are needed to target at-risk pupils.
Unlike many other studies, Pisa is designed to test how well students are prepared for adult life rather than their understanding of the curriculum.
Finland was the top-performing country, placed first in reading and science and second in maths. Hong Kong was placed first in maths.
The UK was not included in international comparisons because data from england did not reach the standard required by the OECD, a government-funded think-tank based in Paris.
The Scottish Executive will publish an analysis of the Scottish results on Tuesday.
Learning for tomorrow's world: First Results from PISA 2003 is available at www.oecd.org