Pupils in London do better at school than the rest of the country, but why? Sam Freedman, who served as an advisor to former British education secretary Michael Gove and now heads an education charity, has called this “the biggest question in education policy.”

According to my research a big part of the answer—almost all of it in fact—lies in the ethnic composition of London’s pupils. And that comes down to higher pupil aspiration, ambition, and engagement among migrants.

There is nothing inherently different in the educational performance of pupils from different ethnic backgrounds, but the children of relatively recent immigrants typically 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; a key part of the “London effect” is the city’s attraction to migrants and those aspiring to a better life.

The best way to measure how a school, or a city-wide school system, fares is to look at pupil progress. This is what I do here, focusing on progress through secondary school. Progress is defined by comparing standardized test scores.

What do we see? First, there is a London premium in pupil progress of just over eight grade points on the United Kingdom's standardized test, where a grade point is the difference between A and B, or D and E. (For an international comparison, this is 9.8 percent of a standard deviation.) This is a huge number: the difference between say getting eight Cs rather than eight Ds.

Ethnic composition matters a great deal. In fact, differences in composition account for all of the gap in the progress measure. If I assume that London had the same ethnic composition as the rest of England, then given the progress of each ethnic group in each place (London/not London), there would be no “London effect.” An easy way to see this is through a simple regression. Apply the “London effect” to all the pupils in all state schools in England, first not taking account of ethnicity and then adding those controls in, and graphs show the effect is wiped out.

There are two important cases where accounting for ethnicity halves the London premium but does not entirely eliminate it. If we consider test scores excluding the various vocational qualifications, such as the assessment in Applied Health and Social Care or that in Leisure and Tourism, there remains a small but significant London effect. This in turn arises because pupils in London enter for significantly fewer of these courses. The implications of this are unclear.

It is certainly not appropriate to simply remove some subject scores from the total and claim that the remainder represents “true” progress. Nevertheless, the fact that London schools systematically entered pupils for less of these qualifications may represent the outcome of a particular policy.

A measure of very high exam performance also yields a small but significant London effect. This may derive from the very high concentration of professional families in London and their high input into their children’s education (which is not captured by a socio-economic variable that just measures eligibility for free school meals).

Many school leaders, politicians, and commentators enthuse about the success of a 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 have improved in a number of ways. But so far at least, catching a reflection of this improvement in the data is proving to be difficult.

It sounds somehow uninspiring and disappointing that the London attainment premium is largely “accounted for by demographic composition” rather than wholly caused by a clever policy. I disagree. It can be seen as a story of aspiration and ambition. There is nothing inherently different about the ability of pupils from different ethnic backgrounds, but the children of immigrants typically have high aspirations and ambitions, and might place greater hopes in the education system.

There is a clear desire among commentators to hang on to the “London effect.” As noted above, this can be done by focusing on a subset of standardized exams—vocational qualifications—and taking that as the outcome measure. But this is currently the only way that that belief can be supported.

The main point is this: We are in serious danger of missing the big issue by focusing on this technical debate over vocational qualifications. In this headlong rush to hang on to the possible effects of a slightly mysterious policy, we are just skipping past a clear and demonstrable achievement of London.

Sustaining a large, successful and reasonably integrated multi-ethnic school system containing pupils from every country in the world and speaking more than 300 languages is a great thing. To my mind, this is what we should be celebrating about the London education system.