In my first job, working for an MP in Westminster, I was often asked where I went to school. I found the question odd; after all, it was unlikely that anyone would know my secondary, an underwhelming, mixed-sex comprehensive in Yorkshire.

After a while, I understood the point – to sort the Harrows and the Dulwiches from the "haven't heard of that"s – and my answer became: "A comp in the north, you won't have come across it before."

At first I felt like an outsider among colleagues who were connected by old school ties, and by family and old boys' networks, but eventually I began to feel relieved. Because, while most were genuinely lovely, almost everyone I met had been impacted negatively by their single-sex education.

Belinda Parmar wrote this week that all-girls schools are good for young women because they remove the pressure to conform to gender stereotypes and the sexism that she believes is rife in co-educational secondaries. I have to disagree.

If sexism was going to flourish in any school you would expect it in a school like mine. Most pupils came from a farming background, where gender divides usually dictate that boys follow their fathers and girls marry off into other farming families. The number of children from difficult backgrounds or those whose parents didn't work was high. The expectations weren't.

And yet the mixed-sex sports lessons, sex education classes and all those other subjects didn't just deposit us rudely on the other side of our A-levels, they helped us to form opinions about the world, recognise differences and grow into well-rounded people.

Because actually, while some of the results in my school were poor, this was probably down to lack of career advice and bad teaching, not because I sat next to rowdy boys in my chemistry class. I'd say poor teaching was the reason that I didn't get enthused about IT and tech, not that I felt as if I couldn't compete.

We did mixed PE lessons until year nine where girls were team captains and led the cross-country. I once broke a boy's nose during a hockey match (the phrase "no higher than the knee" will never leave me now) and not once did I get called a useless girl or bad at sports. I have encountered more jibes and teasing as a young female cyclist than I ever did at school: from adults, not children.

Violent sporting injuries aside, allowing girls and boys to learn and grow up together not separated by gender is more beneficial than sheltering them, despite the possibility of better grades. Co-education prepares you for a world where difference is the norm, not the exception, and teaches you to tolerate and understand it.

It is unlikely that I will ever be part of a friendship group again that includes a boy whose parents left him and his brother in foster care and an illiterate girl whose family were travellers. But those experiences and interactions shake you out of your limited world view and force you to recognise that there are problems bigger than having curly hair and a fringe when straighteners are in fashion.

And credit to the teachers, like those on Educating Yorkshire, who get stuck in and help children to learn to work together. Our sex education teacher was a great example. I'll never forget the lesson where, as young students, she provided the class with two large sheets of white paper and a box of marker pens. "Write all the words you know about sex on there," she said.

Tentatively we uncapped the tips. "Boobs" was scrawled in one corner and covered swiftly by an elbow; a titter rippled through the class. After a while (and no shortage of encouragement from Mrs Waterhouse), other words appeared. Some incredibly offensive.

But instead of passing judgment the teacher stuck both sheets on the wall, nice and high, and read each word out one by one. Then she asked us to look at the words for female genitalia and sex acts, and compare them to the male versions. "See how female words are soft sounding, they contain 's' and 'c'," she explained. "Now tell me about the male words."

Slowly, the penny dropped. "They sound harsh, Miss," one boy called from the back of the room. Others followed suit: "They're like stabbing sounds miss, they sound angry." And so, in one afternoon, one teacher caused her class to rethink their attitudes to sex and gender, without textbooks and without judgments. Without the boys in my class, and without the girls, she could never have taught us that lesson in the way she did.

To assume that children are born with a gendered view of the world, and that the answer is to separate them and teach them in single-sex schools, is defeatist. Yes, society today may have a problem with stereotyping and discrimination, but the next generation must be given the chance to prove those who went before them wrong. If we provide them with the tools to examine the world fairly, they will.