Girls at single-sex schools topped girls from co-ed schools on nearly all academic measures and social and emotional outcomes, according to a new data analysis.
The report, commissioned by the Alliance of Girls' Schools Australasia, looked at Australian and New Zealand Pisa (Programme for International Student Assessment) data, part of an international schools study, from 2015 and 2018.
It included 314 individual measures comparing the responses of girls from single-sex and co-educational schools.
The report found girls' schools excelled on measures of academic achievement in science, mathematics and literacy; academic engagement; teacher effectiveness; involvement in and enthusiasm for science; measures of school belonging; lower prevalence of bullying; and reported higher academic aspirations.
Alliance executive officer Loren Bridge said part of the success was down to girls' schools being able to create learning environments and experiences that shaped girls' self-concept, helping them to overcome the gender biases and stereotypes that are rife in our society.
"In every lesson, every programme and every opportunity — from leadership positions and girl-centric wellbeing programmes, to single-sex physical education lessons and sporting activities — single-sex schools bolster girls' confidence and self-esteem.
"In a girls' school there are no gender stereotypes, no appearance pressures, no sexual harassment, and no barriers to participation — in fact, every aspect of school life is designed and tailored for girls.
"Women are still hugely underrepresented in science, technology, engineering, and mathematics (Stem), so the data that girls from all-girls schools demonstrate notably higher engagement and interest in science and mathematics is an important finding."
St Cuthbert's Year 12 student Nieve Campbell said a major draw for her to change from a co-ed school was the support for her rowing ambitions.
Her previous school had a big focus on boys' sports and teams, such as the rugby first XV, whereas at St Cuthbert's the school recently established its own girls' rowing team.
There was also a similar focused approach in the classroom, Campbell said.
"The focus is solely on girls and the way we learn is different so the teachers can tailor our classes more, which makes it a lot easier to learn."
St Cuthbert's College principal Justine Mahon said the higher academic results in girls' schools was likely due to girls having improved self-esteem and psychological and social well-being.
"In girls' schools they get a non-stereotypical view of themselves and the world, which frees them to feel that they can achieve anything.
"Here they can develop their feminist consciousness, inspire them to go out and challenge social norms, break through glass ceilings, and create a better future for women."
Mahon - who has more than 40 years' experience in all types of schools - said there should be choice, however, and that different young women would suit different learning environments.
"I think overall we, society, need to address those stereotypes, but the patriarchal nature will need to change quite a lot.
"At girls' schools they are able to develop confidence, free of those stereotypes, at a time in their lives which is quite challenging. And as a school we can put strategies in place to foster this."
Ministry of Education spokesman Alex Brunt said it was difficult to determine whether any differences in student outcomes were due to the school being co-ed or single-sex, rather than differences in the students themselves.
Brunt referred to a 1999 longitudinal study based on data collected from students over 18 years, which found much of the differences in outcomes could be explained by differences in prior student achievement or socio-economic background.
This research also points out that there may be other factors that affect the outcomes of students at co-ed and single-sex schools that weren't captured here.
University of Auckland professor of education Peter O'Connor was also cautious of fully attributing academic success solely to the single-sex environment, noting many single-sex schools were private, in higher deciles and were also often religious.
"It is an age-old question, but I think it is a very long bow to draw, when we know socio-economic factors have a significant impact on achievement. So it may be more to do with cultural capital than gender," he said.
"The thing that makes the biggest difference is the number of books that are in the house.
"It is what you come to school with that is the most important determinant, not what you do at school."
Victoria University of Wellington senior education lecturer Dr Michael Johnston said the report results did not surprise him, and mirrored research he'd done on boys' schools in New Zealand.
Over the period from 2013 to 2016, young men from single-sex boys' schools gained NCEA qualifications, University Entrance and New Zealand Scholarship passes in greater proportions than their counterparts at co-educational schools.
However, the reason for advantage was unclear, Johnston said.
"It's not just a socio-economic effect due to single sex schools tending to be in higher socio-economic strata, even though they do. The work I did on boys' schools showed that the single-sex advantage held true at all socio-economic levels."
Other factors could include the "selection effect", where parents making the choice could be more committed to supporting their children's education; and that teaching in single-sex schools could be more focused for one sex or the other - although this was also not clear, Johnston said.
"My best guess is single-sex schools tend to have a more traditional ethos that engenders price in the school, resulting in students engaging more fully.
"I think the Ministry of Education ought to undertake or fund research to find out why students in single-sex schools tend to do better - it might be something we could translate into co-ed settings."
However, Johnston said the academic outcomes were not everything.
"That [single-sex] environment is good for some students but others thrive in a co-ed environment.
"Notwithstanding what I know about the reliability of the single-sex school advantage to academic performance, when my daughters are of secondary-school age, I'll be sending them to a co-ed school.
"Academic performance is very important, but so is being socialised to live in a world in which men and women interact in all walks of life."