I really am not certain, and am unaware of any reliable data on the matter. What I do know is that the figure is small, and should be much higher. The utterly counter-productive and damaging wall between both education sectors needs to be ripped down stone by stone, and as quickly as is feasible, if the nonsense of the last century is not to be repeated.
In recent weeks, I have written about what the independent sector can learn from state education. Today, I write about seven areas where I think the state can learn about what has made the independent school sector in Britain the most successful in the world.
Small class sizes has to be a key factor (as indeed is the far smaller unit size of each school). Education research tends to show that class size makes little difference to results. This is misleading nonsense. Ask any teacher or any pupil who sits in a class of 20 to compare the learning experience with being in a class of 36.
Teachers are better able to control the class and its learning, to know and interact with each pupil individually, and to give quality time to the monitoring and marking of their work. Pupils are able to speak more and have their individual needs catered for. They grow more, not only academically but also socially. It may not be obvious to researchers but it is obvious to me.
As was highlighted again during the unions' conference season, discipline is of paramount concern to teachers. Those in independent schools are able to teach their subjects to generally biddable children and have to spend far less time on crowd control or being humiliated by unpleasant confrontations than are their state colleagues. More independent-school children come from homes where they are socialised into good behavioural patterns and respect for authority. But independent school teachers are also able to be stronger disciplinarians, and they receive firm support from the school's leadership all the way up to expulsion. This ultimate deterrent is a core guarantor of good behaviour, even when little used, and it was folly to diminish its threat in state schools.
House systems, house pride and competitions underpin everything that happens in independent schools. Some state schools are introducing them, as my school is in its academy in Wiltshire. This model of pastoral care and "ownership" could easily be emulated everywhere.
The vision of "education" is far broader in independent schools, with almost all children able to act in a play, sing in a choir, play in a team, have a leadership role, and so on. How odd that the more socially privileged should have opportunities denied to those with less.
Parental involvement is fundamental to independent schools. It can be over-intrusive, but a strong head will delineate the boundaries. Parents give so much to the schools - not least helping their children deeply to value and respect it.
Teachers are generally treated with great respect, and particular care is taken of their welfare and wellbeing. At best, they are treated as colleagues in a college rather than mere employees in an impersonal organisation. Nothing matters more in a school than teachers.
Finally, independent schools have independence. The greatest achievement of Labour in the past 10 years - and it is being continued by Ed Balls - is to give state schools more freedom than they ever enjoyed before. Every individual and every institution flourishes where they are given respect and autonomy. This is the greatest single secret of the success of the independent sector, and - unlike some of the factors above - it is one that can be spread without spending a single penny.
Anthony Seldon, Master, Wellington College, Berkshire.