The University and College Union (declaration of interest: I am a member) have publicised ONS figures showing that the proportion of people with no skills varies dramatically by parliamentary constituency.
The data are based on samples, so the margins of error are large. But, taken as a whole, UCU are right: some areas have a lot more skilled people than others. But they are wrong to deduce from this that schools are much better in some areas than others. The issue is not that schools in Glasgow North East are terrible, and that schools in Brent North are wonderful. The issue is that people don't move randomly. (The government's response was just as mistaken).
Put simply, people with qualifications generally earn more money and then choose to live in affluent areas. People with fewer qualifications generally earn less money and then choose to (or are forced to) live in poorer areas. So we get concentrations of skills. This is particularly true within cities.
There is a huge amount of evidence that children whose parents are poor generally do worse at school than children whose parents are rich. So no doubt school results are worse in Glasgow North East than the national average. But children whose parents are typical for Glasgow North East do pretty much as well (or as badly) wherever they live. There is virtually no evidence that mixing poor kids and rich kids leads the poor kids to do better at school.
That is why it is important that we concentrate on people and not places. We need our schools to work for poor kids, whether they are the only poor kid in the class, or in a class with 29 other poor kids. That is the challenge - and the pupil premium, pitifully small though it currently is - is almost certainly the right way forward.
Follow Tim Leunig on Twitter: www.twitter.com/timleunig