Gentrification, ambitious teacher spouses, academies, an economic boom, Teach First and government policies have all been among the possible explanations for the incredible rise of London’s state schools.
But now new research suggests there is a much simpler, single, reason for a decade of improved GCSE results in the capital: London schools do better than the rest of England because they have a higher proportion of ethnic minority pupils.
The report by the Centre for Market and Public Organisation (CMPO) at Bristol University argues that the diversity of the capital's population is a key reason for the "London effect" because ethnic minority pupils tend to achieve higher grades than those from a white British background.
The findings cast doubt on the wisdom of politicians from all three main parties who have suggested that replicating key aspects of the London Challenge school improvement scheme could trigger similar improvements in the rest of the country.
The study 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, taking into account prior primary school attainment,
The findings showed that London state school pupils score around eight GCSE grades higher than those in the rest of the country, relative to their results aged 11.
The "London effect’ in pupil progress therefore equates to the difference between gaining eight A grades compared to eight Bs, or eight Cs compared to eight Ds.
But it disappears once pupils' ethnic background is taken into account, the study finds. It suggests that there is a similar situation in Birmingham which also outperforms the rest of the country and has high percentages of ethnic minority pupils.
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 confirms that white British pupils achieve the lowest GCSE results in relation to their attainment at the end of primary school. They make up just a third (34 per cent) of year 11 pupils in London, compared to 84 per cent in the rest of England.
But it suggests the difference in performance between ethnic groups is more about motivation than inherent ability.
“The children of immigrants typically have high aspirations and ambitions, and place greater hopes in the education system than the locals do," according to the report 'Understanding the success of London’s schools’.
Researchers also looked at the impact of children of recent immigrants and found evidence showed this played a part in London’s performance, as well as ethnicity.
In Newcastle, 11.8 per cent of the population was born abroad and arrived to the UK before 2000, compared to around 34.7 per cent in London.
The study found this contrast accounted for an average per pupil difference of around 15 GCSE grades between the two cities, 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."
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Primary schools behind boost in London GCSE results June 23 2014