Sir Peter Lampl, chairman of the trust, has called for a radical review of social mobility to see what is going wrong.
Some politicians have nostalgically blamed the near-demise of grammar schools for the problem, as they were seen as a "leg up" for poorer children who were able to pass the 11-plus. Although they undoubtably increased social mobility for some, recent figures show the remaining 164 grammar schools are dominated by the middle-classes, or the relatively well-off.
David Jesson, professor of economics at the University of York, found that only 2 per cent of grammar school pupils were eligible for free school meals, compared to an average of 12 to 14 per cent of children in the same areas.
The Labour government decided against expanding systems which select a handful of talented underprivileged children for top private schools by scrapping the assisted place scheme. Instead, it has turned to initiatives such as academies to improve achievement in disadvantaged areas, and tried to reverse the polarisation of schools by class through admissions codes.
But critics have come down hard on the approaches, saying that the parental choice agenda, combined with catchment area decisions, can still favour the middle classes.
To try to tackle the attainment gap, Labour has been investing billions in early years education and childcare. Pam Sammons, a principle investigator on the Effective Provision of Pre-School Education research project, said she did not think the investment would close the gap, but it could narrow it. "We've only had access to state education for around a century, and access to pre-school for most in the past 15 years, so it is really too early to tell what will happen," she said.
Professor Sammons emphasised that other key factors in a child's early achievement were cultural aspects, such as their mother's qualifications, and the learning environment in their homes.
Dr Elliot Major agreed: "To be fair to Labour, they have invested in early years. But we always think that more could have been done - putting more into access schemes for universities, and working with 11-year-olds before they have a chance to drop out, for example."
The sociological theory which was popular in teacher training in the 1970s and 1980s has largely been replaced by a focus on specific identified issues, such as the performance of white, working-class boys. Diane Reay, professor of education at Cambridge University, has called for a return to the 1970s approach, in which trainee teachers read key texts providing an understanding of the workings of social class within schooling.
In a 2006 paper, she acknowledged that there were some "incredibly impressive" new teachers, sensitive to social class differences.
However, Professor Reay added that a focus group of teachers revealed some "ill-informed and prejudicial views" about working-class parents not caring about the schooling or the success of their children.
"Initial teacher trainees are left ill-informed and ill-equipped to broach, let alone tackle, the greatest problem the education system faces - that of working class educational underachievement, alienation and disaffection," she said.
IT HELPS TO BE THE CHILD OF A LAWYER
Likelihood of gaining five or more GCSE grades at A* to C, by parents' socio-economic class (with example occupations)
Higher professional: 77% (Doctors, lawyers, dentists, professors)
Lower professional: 64% (Teachers, journalists, nurses, actors)
Intermediate occupations: 52% (Secretaries, airline cabin crew, photographers, firemen)
Lower supervisory: 35% (Train drivers, plumbers, foremen)
Routine: 32% (Bus drivers, waitresses, cleaners)
Source: Pupils in England and Wales, Youth Cohort Study, DCSF, 2002.