This is the chance of a child at school in England passing five good GCSEs if their parent is a doctor, or a waitress. Can anything be done?
Part Three: Class - The sociology of the 21st-century classroom.
Former prime minister John Major, the son of a circus entertainer, in 1990 promised to work to create a "classless society". Tony Blair, who followed him in 1997, declared "we are all middle class now".
But with the prospect of Tory leader David Cameron and his cadre of Eton College alumni winning the next election, Britain's future leadership is looking distinctly posh.
As white working-class boys continue to fail academically in their thousands, a public school education, followed by Oxbridge, still seems like the ticket to the top.
Local accents may not set you back as far as they used to, but after 10 years of Labour's striving for a so-called "meritocracy", those born without the advantage of wealth and social privilege still, statistically, face an immense battle if they wish to climb the social scale.
A young person looking for a top job in this new meritocracy seems to need an ever-increasing pile of excellent exam results and access to a leading university. For the 60 per cent of pupils over the past 10 years who failed to gain at least five GCSEs at C grade, including English and maths, this is an insurmountable barrier.
A recent study for the Sutton Trust, which supports educational projects for young people from non-privileged backgrounds, found that despite initiatives, social mobility in the UK has stalled. People born in 1958 had a higher chance of earning more than their parents than those born in 1970. Children born after 1970 have seen no subsequent improvement in mobility.
One of the report's authors, Stephen Machin, professor of economics at University College London, said the recent expansion of higher education had benefited middle-class teenagers rather than those from the poorest 20 per cent of households.
In the 2006 Pisa tests, which compared the knowledge and skills of 15- year-olds in 43 countries, England could be proud of having comparatively few pupils in the bottom achievement level. But it did have one of the largest gaps in science attainment between children from families of very low and very high socio-economic status.
The socio-economic status of a family has been found to be a far more important factor in a child's future academic achievement than other areas of inequality, such as gender or race.
And the gap begins to widen at a startlingly young age. The Sutton Trust research, published in December, found the brightest children born into the poorest families in 2000 are already being overtaken in test scores by the least academic children from rich backgrounds.
If a representative group of 100 children took cognitive tests, a three- year-old in the brightest group from the poorest households would be in 12th place. By the age of five the child would have dropped back to 35th place. Meanwhile, a child in the least able group from the richest households would have risen from 85th to 55th place.
Lee Elliot Major, director of research at the Sutton Trust, said: "The social class attainment gap is still very significant. Anyone who thinks that class doesn't matter has got their head stuck in the sand.
"Education has become more important than ever. The premium on having a degree has gone up; the premium on prestigious universities has gone up. The stakes are a lot higher over all.
"Economists looking at parental income and sociologists looking at
How pupils perform at school can be heavily influenced by their gender, race and social class. Each factor has been the subject of intense debate.
The gaps between the performance of different groups has also led to public concern and a multitude of national and local initiatives.
In this special series, The TES examines each factor in turn to see how it affects teachers and pupils. In the fourth and final part, we will explore how the factors overlap and the challenges that this creates for schools.