Across the home counties, behind the cricket matches and elaborate speech days, a war has broken out. Single sex or co-ed? Which is better? Which is right?

While Professor Alan Smithers’ recent findings expose the “myth” of the advantages of single-sex education, psychologist Leonard Sax argues that boys and girls should be educated separately.

Thirty years ago there were 2,500 single-sex schools in the UK independent sector; today 400 remain, fighting increasingly pitched battles to maintain their hold on the market.

A striking element of this trend has been the number of boys’ public schools which have opened their robust and intrinsically male doors to girls in the past 30 years, including Marlborough, Bradfield, Rugby, Uppingham and Oundle.

Most recently, Wellington college has joined the co-ed cohort. It will be welcoming 13-year-old girls from September. I know. Last year, as a member of Wellington’s common room, I argued passionately that Wellington should eschew its macho, aggressive past and go co-ed.

Having revelled in teaching generations of rugby-obsessed, gangly, gawky, charming boys the delights of Shakespeare, Milton and Philip Larkin over the past 12 years, I decided to move this year to Prior’s Field school, a girls-only day and boarding school in Surrey. I still live at Wellington, where my husband is a housemaster, but I am now enjoying teaching English and running the sixth form in a predominantly female environment.

So I find myself somewhat uncomfortably straddling the gap between the two camps. I see what a benefit girls are to Wellington and to schools like it.

They do civilise and soften, they do inspire boys to raise their game. The school is undeniably richer (in every sense) for their presence.

But at Prior’s Field, I see the other side. I see girls revelling in un-selfconscious learning. I see them enjoying success and challenge without aggressive competition. I see a far more pervasive academic ethos and an unquestioned assumption of the need for effort and for integrity in the classroom. And that is an exciting prospect for a teacher and, I suspect, for a parent.

Parents are also influenced by league tables. The extraordinary success of single-sex schools in those tables seems to suggest greater academic achievement without the distractions of the opposite sex. The figures speak for themselves: an Independent Schools Council extrapolation of Department for Education and Skills data produced last October, found that the percentage of A grades at A-level among Girls’ Schools Association members was 51 per cent, while girls in co-ed independent schools scored 39.8 per cent A grades.

Critics will attribute this to the highly selective entry of single-sex schools but this is a circular argument. Parents choose to send their very able children to these schools because they are academically impressive.

Anthony Seldon (the dynamic new master of Wellington) wants to “celebrate difference” and to increase variety. But the serious decline of single-sex schools (the vast majority of them boys’ schools), clearly mitigates against variety. The single-sex market is being squeezed and I feel surprisingly militant on behalf of the girls’ schools. I was a pupil at North London Collegiate school. But, next to the vast majority of wealthy boys’ public schools, it cannot compete in material terms.

The truth is, most girls’ schools have very little in their arsenal to combat the money, facilities, tradition, history and power of the boys’

public schools. The public schools have become the out-of-town megastores stealing custom and market share from the small, specialist, high-street shops.

Whatever the rights and wrongs of co-education, there is something galling about these Goliaths wading into the fray, because they could not make it as single-sex institutions any more. The girls’ schools will need to have their catapults ready.

I don’t suppose there are any easy answers. As deputy head of Prior’s Field next year, I will be working hard to ensure that parents hear what I have discovered about the joys of working in a girls’ only environment.

At the same time, I will be rooting for Wellington. I have no doubt that it (and Dr Seldon) will make an outstanding success of its particular brand of happy co-education. But I will stand by the right and vital importance of girls and boys to be educatedseparately - if they choose to be.

The staggering differences in the way my own son and daughter play, work and socialise, suggest the importance of dealing with them in different ways. They require it. Why will they stop requiring it when they get to senior school?

The real issue at the heart of all of this is the question: what makes a good school and what makes a school right for one’s own children? And that’s an entirely individual decision. Is it about co-education as opposed to single-sex? Possibly not. It’s probably more about visiting a variety of schools and deciding which one “feels right” for your son or daughter.

Which school do your children think they can do business with? Because nine times out of 10, that will be the right one!

Jane Lunnon, is head of sixth form, Prior’s Field school, Godalming