Choosing a secondary school is hard, even for the most competitive parent-child teams. There are academic queries, state or independent considerations and swimming pool conundrums. Then there is the seemingly eternal question of co-education or single-sex.

It’s not so important, argues Adam Pettitt, headmaster of the independent Highgate School, which back in 2003 took the decision to end 400 years of single sex education by letting in girls.

“I feel that Highgate has much more in common with good single sex schools than it has differences,” he says.

However Helen Pike, headmistress of South Hampstead High School, independent girls’ school and Girls’ Day School Trust member takes a feisty approach.

“Lots of research shows that boys benefit from having girls around. Girls do not benefit from having boys around in the same way.”

It’s a debate worth having: single-sex secondary schools are becoming increasingly scarce, so parents are more likely to make a positive choice to send their child to one.

The decline in single sex schools in the last 25 years has affected boys’ schools more profoundly. In 2013, little more than 400 maintained and private mainstream boys’ schools existed, compared with more than 620 girls’ schools.

Some traditionally all-boys’ schools, such as the independent University College School, have welcomed girls into Sixth Form in the past few years.

Mark Beard, headmaster of UCS, reasons: “Post-GCSE, the boys have matured quite a lot – most of their adolescence has fallen away and that is a good time for the girls and the boys (to be educated together).

“The girls who come to us are virtually all from single sex girls’ schools themselves. At that point, the students have the confidence to be able to work together, rather than against one another.”

“When I was teaching about 20 years ago, I would have said that boys were at their most adolescent at about 14, maybe even 15. Nowadays I would say, you might expect to see that more in Year 8, rather than in Year 10.”

Pike agrees, from her experiences at South Hampstead, that children are growing up faster. “In many ways, it’s a tough time to be a girl. For example, I often say that Year 8 is the new Year 9 –they seem to get that much older, that much younger – and parents increasingly call on us to help them with things like body image and social media; and the two things are linked.”

Outside of pastoral care, academia is another key argument for single-sex schooling. The Institute of Physics, examining data from maintained schools in 2011, found girls were almost two and a half times more likely to go on to do A-level physics if they came from a girls’ school.

Carol Jones, head of Hornsey School for Girls, a member of the Association of Maintained Girls’ Schools backs up these findings.

“Girls who attend single-sex girls’ schools tend to be more successful academically – and that is particularly true of girls who were studying what was hitherto defined as male-dominated subjects.

“Girls at girls’ schools are encouraged and supported to follow through on higher-order subjects – science, maths, engineering, technology.”

For his part, Beard believes boys can be more creative without girls. “If girls are present sometimes boys might act up in a certain way or they may feel inhibited to explore a particularly creative side of life.

“What I’ve seen in UCS and at other boys’ schools is that things like the calibre of drama and music have been absolutely phenomenal. That is not necessarily what you get in co-educational schools.”

He believes the school’s liberal atmosphere, “allows the boys to be naturally inquisitive about the world around them. Being all boys together allows more of them to be confident about exploring ideas – and to develop their moral fibre.”

Pettitt observes that co-education at schools such as Highgate can also play a part in encouraging children to take up subjects traditionally dominated by the opposite sex.

“Co-education is a powerful inoculation against using gender to determine a pupil’s life chances. We must confront stereotypes: some subjects are seen as male or female.

In school we can use the curriculum to right the bias; ensure both boys and girls are seeing women and men teaching physics and maths; explain why it’s important and good that dads take parental leave and work-part-time; ask hard questions if the proportions doing sciences or humanities tip one way or other; invite female speakers from business, industry and universities.”

He endeavours to create a sense of equality within the school: “As long as the world beyond the school’s gate remains unequal, we will face challenges internally that reflect those inequalities.”

Partly given these inequalities, Pike argues that single-sex schools such as South Hampstead are the finest preparation for children going into the world. She says girls’ schools were set up partly to champion female rights.

“Those schools were very self-consciously working for a group who were unequal in all sorts of ways, and still are – to a lesser extent, but to an extent.

“One of the things girls’ schools have done well is to keep building on and modernising – and keep relevant that distinctive identity and that distinctive focus on the concerns of girls. We all go out and work together, in the world, but children aren’t adults – you can focus on them at that really important developmental time.

“And, let’s face it, my school isn’t a convent, they’re in North London, they’re going out on the weekends.”

Perhaps these words serve as a reminder that the choice between single-sex or co-educational schooling isn’t everything: both girls and boys will also educate themeslves outside the school gates.