Fennel - I would agree that the girls in single sex schools are the same girls who would be successful in mixed school. As I've said elsewhere, the fundamental problem with establishing the benefits or otherwise of single sex education for girls lies in the fact that single sex schools are overwhelmingly private or selective schools. I haven't been able to find out how many single sex comprehensive schools there are in the UK, but as far as I can see there are only around 400 single sex state schools including grammars.
Its also fair to say firstly, that the benefits or otherwise in terms of educational achievement are contested. As far as I can see, some studies conclude that girls with lower levels of educational attainment benefit most from single sex education, other studies conclude that there is no 'value added' effect and the differences are linked to differences in the population of girls attending single sex schools.
Secondly, IMO the benefits of single sex education for girls are not really about numbers of GCSEs or A levels gained, or even what subjects those results are in (though of course it is important). They are more about carving out a place for women where they are primary in importance. As such it isn't surprising that they get a lot of knee-jerk antagonism.
I suppose its a bit like Mumsnet - would we all come on here and speak with the same freedom if MN were equally populated by men and women? I know that there are men on here (and of course it isn't obvious who is male and who is female), but essentially it is a women's space. And couldn't the same objections be made - what sort of women need to come to a board like this for support; are we all women with low self confidence, poor body image, or strong libidos (or indeed maybe all three)?
Or, just perhaps, are we women who can perfectly well function in mixed sex groupings in the 'real world' but who like from time to time to operate in a society where we are not the second sex?