Teenagers in London score higher GCSE results than those in the rest of the country because the capital's schools have a higher proportion of pupils from ethnic minority backgrounds, according to research.
The new study argues that the diversity of the city's population is a key reason for the "London Effect" - a term that has been coined to describe the high levels of achievement and progress among London students.
Ethnic minority pupils tend to achieve higher GCSE grades than those from a white British background, researchers suggests, and as they make up a large proportion of the student population in London's schools, this help to boost the capital's overall exam score.
The study, by the Centre for Market and Public Organisation (CMPO) at Bristol University, analysed 2013 GCSE data for all pupils in state secondary schools in England.
It looked at each student's GCSE points score across their eight best subjects, against their attainment in exams taken at the end of primary school and examined other measures such as the percentage of students score at least five C grades at GCSE. Researchers also looked back at results over the previous decade.
The findings show that pupils in London state schools score around eight GCSE grade points higher than those in the rest of the country, relative to their results at age 11.
This is the difference between gaining eight A grades compared to eight Bs, or eight Cs compared to eight Ds.
But once pupils' ethnic background is taken into account, this "London Effect" in pupil progress disappears, the study suggests.
White British pupils tend to achieve the lowest GCSE scores against their attainment at the end of primary school, compared to those from ethnic minority backgrounds, previous CMPO research has found.
This group also makes up just a third (34%) of Year 11 (15 and 16-year-olds) in London, while they make up around 84% of this school year group in the rest of England.
Report author Professor Simon Burgess said: "We know that ethnic minority pupils score more highly in GCSEs relative to their prior attainment than white British pupils. London simply has a lot more of these high-achieving pupils and so has a higher average GCSE score than the rest of the country.
"Many policy makers, school leaders and commentators enthuse about the major policy of the time, London Challenge, and view it as unambiguously improving schools in London. This unanimity carries weight, and no doubt London schools were improved in a number of ways. But so far at least, catching a reflection of this improvement in the attainment data is proving to be difficult."
The study concludes that the findings can be seen as a "story of aspiration and ambition".
"There is nothing inherently different about the ability and performance of pupils from different ethnic backgrounds," it says.
"But the children of immigrants typically have high aspirations and ambitions, and place greater hopes in the education system than the locals do."
Researchers also looked at the impact of children of recent immigrants, rather than looking at ethnicity and said that evidence shows that this also plays a part in the "London Effect".
In Newcastle, around 11.8% of the population was born abroad and arrived to the UK before 2000, while in London 34.7% of the population did so. Comparing these two cities, there is a difference of around 15 GCSE grade points in pupil progress, with London ahead.
Professor Burgess said that while there are no key differences in the abilities of pupils from different backgrounds, the children of relatively recent immigrants often have "greater hopes and expectations of education, and are, on average, more likely to be engaged with their school work."
This is not by chance of course," he added. "A key point about London is its attraction to migrants and those aspiring to a better life."
The report is the latest in a number of studies to look at the success of the capital's schools.
In July, a paper by the CentreForum think tank concluded that the life chances of thousands of children would improve if schools across the country reproduced the results gained by poor pupils in London, according to research.
It found that schools in London are adding up to half a grade more progress in each subject than those in the North East, Yorkshire and the Humber and the East Midlands, and also outperform those in other regions.