In the second part of a new series Caroline Jordan, headmistress of Headington School and vice president of the Girls’ Schools Association, explains why only boys' schools go co-ed at sixth form.

Single-sex schools, I am told, are a thing of the past.

A throwback to an old-fashioned way of thinking with no relation to life in the real world.

It is certainly true that there are fewer girls’ or boys’ schools than there were 20 years ago.

If you were to believe the press, barely a week goes by without you hearing about another girls’ school which has opened the doors to boys in its nursery or a boys’ school which is welcoming girls to its sixth form.

I, however, would argue that single-sex schools are even more relevant and more important than ever.

While educational policy seems to be in a perpetual state of flux, single-sex education remains a constant in the background and I believe that, for girls in particular, it is vital it continues to flourish. While some schools do choose to go co-educational, either in entirety or for a portion of the student body, what you do not see is girls’ schools going co-educational at sixth form level.

The reasons for this, for me, go to the heart of what makes girls’ schools so vital.

Last year, research from educational data analysts SchoolDash showed what many of us in the sector already know – girls in single-sex schools get better results.

That advantage remained even when other factors, such as socio-economic background or selective intake, were taken into account.

Not just the brightest, wealthiest girls – pupil progress as a whole was higher and poorer pupils got better results than in a mixed setting.

Boys in single-sex schools also performed better than in mixed – but there was no significant advantage when other factors were taken into account.

This research is backed up by long-term tracking studies, according to the Centre for Longitudinal Studies at the UCL Institute of Education.

As well as getting better results, researchers found girls in a single-sex setting were more likely to take male-dominated subjects such as maths and science and had slightly higher wages in later life. We are still in a world where women are paid less for the same work, where even organisations such as the BBC are embarrassed by the scale of their gender pay gap.

This is important in a very real sense.

So why do girls do better in a single-sex environment? In a girls’ school you can play to girls’ strengths.

In general, that means more of a focus on collaborative, discussion-based learning.

Boys can dominate a discussion and force themselves into leadership roles in a mixed classroom. When it’s just girls, they choose subjects they might have avoided as stereotypically boys’ subjects and are challenged to take the lead.

If there are no boys to fill their pre-conceived roles then you can be wonderfully free from gender stereotypes – being a girl can mean much more than someone else’s narrow interpretation. Excellent schools, both single-sex and co-educational, will have teachers who are expert at bringing the best out in their students but in a single-sex school they are able to focus on the best possible methods for their charges, particularly important when we are looking at the academic rigour of A-Levels or IB.

There are, of course, social and developmental reasons why some girls may wish to be in a mixed sixth form once they finish GCSEs.

Indeed, every year we say goodbye to some of our girls as they embark on a co-educational adventure but increasingly every year, we welcome more and more of them back when they discover working alongside boys in the classroom was not quite what they had expected.

So really, why would we invite boys into our sixth form classrooms? (Apart from, of course, the frequent occasions when we invite them to join us for debates, workshops, music or drama events). We want to give our girls every chance to shine – to fight gender stereotypes and the gender pay gap and to do the best they possibly can.

They don’t need a boy sitting next to them for that.