There is something rotten in the state of English state secondary education. Despite continuing rises in spending, stable pupil numbers and a new government emphasis on secondary schools, the gap between what children from rich and poor backgrounds achieve at GCSE is as wide as it has been for a generation. In 2006 it was already wider than in any other advanced economy. In the past year it has expanded, astonishingly, by ten percentage points. As a result, pupils in South Buckinghamshire, say, or parts of Cheshire, are up to 43 per cent more likely to achieve at least five good GCSEs than those in Tower Hamlets or Co Durham.

This is not an indictment of the state sector as a whole, nor of all this Government's efforts to reform it. Standards have risen twice as fast in wealthy areas as they have fallen in deprived ones, proving what can be achieved where good state schools harness the support of middle-class parents with high expectations for their children. But the figures are dismaying nonetheless, and especially for Labour, as a vital indicator of social mobility. In some of the country's most deprived areas, that mobility is now stagnant or declining. In the language of old Labour, the class divide in education is as raw as ever.

The data on which we report today has been compiled by the Conservatives. But Ed Balls, the Secretary of State for Children, Schools and Families, would be unwise to attack it on that basis. The figures accord with others released in the autumn by Ofsted. Gordon Brown has acknowledged that the attainment gap in schools is widening and unacceptable, and has directed ministers to redouble their efforts to close it. Unfortunately, their strategy has so far failed.

A central plank of that strategy remains the building of city academies, intended as centres of excellence where previously “sink” schools were the norm. This newspaper has supported the new academies consistently, both as magnets for private sector investment and expertise, and as part of the solution to the chronic undersupply of good school places of any kind. Indeed, the academies' chief problem is their scarcity, which is why the Government's pledge to build 200 of them by 2020 is welcome. If fulfilled, it could help to make a reality of parental choice in secondary education. But in the meantime, that choice, for most, is notional at best, with middle class children overrepresented in the best state schools, thanks to their parents' ability to buy their way into sought-after catchment areas and take advantage of selective admissions policies.

This “colonisation” of good schools by ambitious parents is, in the Tory analysis, a main reason for the attainment gap. Hence their proposals to narrow it by accelerating the building of academies and introducing cash incentives for good schools to seek out children from disadvantaged backgrounds. But cash, whether attached to buildings or individuals, cannot be the basis of a sound strategy for making education the social ladder that deprived pupils need. As Sir Peter Lampl, of the Sutton Trust, wrote recently: “Good schools ... are about leadership, discipline, ethos and - crucially - teaching.” When Mr Balls promises “to focus on all the needs of every single child”, and to reward schools for providing “extended services” that have little to do with core curriculums, he is in danger of forgetting this. He should focus on bringing great teaching to every child in every classroom, new or old. Without that, nothing else counts.