Bernice McCabe has been going through some of her old school reports lately. They date from a time in her early teens when she was losing interest in lessons because she had a new boyfriend, and double Physics no longer seemed so appealing.

Most of the reports have something negative to say, whether it be criticising her untidy handwriting or her propensity for letting her hair fall across her face in class.

“There’s no encouragement at all, is there?” McCabe says, as she leafs through them and she sounds rather shocked. As headmistress of the North London Collegiate School for Girls for the last 20 years, McCabe has always made it a priority to encourage her students.

Now on the verge of stepping down, the 64-year-old describes herself as “an optimist”, and believes the quality and professionalism of teaching has actually “improved” over the four decades she has been a teacher.

There’s no doubt that the teaching at North London Collegiate, one of the top single-sex independent schools in the country, is pretty impressive. The school, set in a white-bricked mansion in a leafy enclave in north London, is consistently at the top of the league tables, and alumnae include the actress Rachel Weisz and American Vogue editor Anna Wintour.

Last year, more than 98 per cent of pupils at GCSE and 87 per cent at A-Level were graded A* or A. So what’s the secret?

“Teachers who never write anyone off,” says McCabe, who is also the co-director of the Prince’s Teaching Institute, set up by Prince Charles to re-engage teachers with their subject and their students. “I believe in inspiring pupils and in building their resilience so that when things don’t go as expected, they can cope.”

When McCabe was first appointed in 1997, she took a group of sixth formers into her office and asked them what they wanted from their headmistress. One of the prefects said that she wanted to know that if she left school, went to university, had a career and also a family, that she didn’t need to feel guilty about it. McCabe’s response was pragmatic.

“I suppose the point I’ve always tried to make is that you can have it all, but not necessarily at the same time, and you won’t necessarily be able to be completely free of guilt or regret. Choice is wonderful… but with those choices, inevitably come regrets or feelings of guilt about what you haven’t done or aren’t doing.”

Does McCabe feel guilt?

“Yes, I do. Women are quite good at feeling guilty. I feel guilty about not spending enough time with the girls or the staff, or not spending enough time at home with my own family. I have a niece and nephew I love and I’d like to spend more time with them. I do think we beat ourselves up.”

McCabe herself has never had children. She has been married twice, but it never seemed to be the right time to start a family. “I don’t really know the answer to it,” she says.

She admits that “one feels a little bit wounded” after two failed marriages, so “I committed myself to my career. It’s not that I’ve chosen not to have children, but my father did once say he could never see me pushing a pram.”

I can half see what he means. McCabe must be the most glamorous headmistress I’ve ever met. Her hair is freshly blow-dried, her make-up impeccable and her nails painted a becoming shade of peach.

Today, she is wearing a loosely tailored white jacket, a silk blouse, black trousers and spiky heels. Her light-filled office is dotted with cut roses in crystal vases and on the back of the door hangs a suit carrier from Margaret Howell. The whole impression is one of casual fragrance. Sitting opposite her, I feel like a dowdy housekeeper being interviewed for a position by Jackie Collins.

She has always been interested in fashion and tells all the girls that it’s perfectly acceptable to be interested in both make-up and academia. On Tuesdays, she welcomes a group of pupils to her office for tea, and generally “we have a good gossip about nails… They always notice what I’m wearing or what nail polish I have on.”

When McCabe was an English student at Bristol University in the early 1970s, she used to wear a pair of green knee-high platform boots to tutorials, “and my tutor was very Women’s Lib, very feminist, and she used to comment, saying I couldn’t wear that. She really disapproved, and I always hated that. I thought it was quite repressive.

“The visual thing is important to women. I always wanted the girls here to feel they could enjoy clothes and that there was no one way of being.”

Of course, it’s easier to impart these things at an independent school, where fees are £19,635 a year and the teacher-student ratio is 1:9. McCabe, whose first job was at Bristol Comprehensive and who spent years in the state sector, insists that she has met “hundreds” of brilliant teachers along the way, but she does worry that middle-class parents will no longer be able to afford to send their children here if prices keep rising.

“I don’t think I would have been able to come somewhere like this without a bursary,” she says. “There is a concern about independent schools pricing themselves out of existence.”

Her next role after stepping down at NLCS is overseeing the school’s expansion internationally: there is already a campus in South Korea, and NLCS Dubai is set to open this September, with Singapore following shortly afterwards. The fees from these schools will enable more money to be put into bursaries, McCabe says, so she sees it as a virtuous circle.

Bernice McCabe: 'There is a concern about independent schools pricing themselves out of existence'
Bernice McCabe: ‘There is a concern about independent schools pricing themselves out of existence’ CREDIT: ANDREW CROWLEY

Although her mother was a teacher, McCabe never intended to be one herself. She fell into a PGCE at Bristol because she couldn’t think of anything better to do, but it’s clear she has found her vocation.

As well as being a headmistress, she has been instrumental in making exams more rigorous, having served on former education secretary Michael Gove’s advisory committee on curriculum reform which got rid of the AS-Level and made GCSEs more challenging. Despite opposition from the state sector, who accused Gove of harking back to the 1950s, all these reforms are still in place.

McCabe remains a firm believer that teachers should teach “subject, not syllabus” and that girls, who can struggle with self-confidence, are not hampered by negative language.

“I hate the phrase ‘glass ceiling’,” she says, “because it’s limiting. It’s ‘Oh, gosh, we’re under threat; there’s going to be things we can’t do.’ Just go out and be wonderful!”

As life advice goes, it’s probably rather better than telling a young girl to keep her hair out of her face.