Pat McGuckian views Northern Ireland's selective schools system as "tantamount to child abuse".
Attracted to the reputation of St Patrick's High as a successful mixed-ability school in a province where grammars reign, she took the job as principal in 2011. But what she found was a school that ran its own internal selection system and consigned hundreds of students to failure.
So Ms McGuckian asked her staff to gamble the school's good reputation in the community in Keady and provide a genuinely comprehensive education.
Today, it is the highest-performing non-grammar school in Northern Ireland. Eighty per cent of students gain five GCSEs at A*-C including English and maths, up from 51 per cent - a transformation that earned St Patrick's the title of secondary school of the year at last month's TES Schools Awards. At the average non-grammar in Northern Ireland, the figure is just 44 per cent.
"There's a lot of mystique that goes on with grammar schools," Ms McGuckian says. "Some people do exceptionally well. But about 40 per cent of our pupils in Northern Ireland leave school without GCSEs in maths and English. Four out of 10. That's scandalous.
"I do feel, really - and I'm sorry if this is too emotive - that the system in Northern Ireland is tantamount to child abuse. It's flawed educationally.
"There's no research that supports a child's ability being fixed at 11. It's a 19th-century idea that children should be segregated and categorised at 11."
The banding system that the 1,000-pupil school used steered lower-ability children on to vocational courses in motor-vehicle maintenance or child development. Students found themselves separated from their primary school friends based on results.
"The school had built a reputation in the community, and after four weeks I was coming in to say I wanted to try something completely different," Ms McGuckian says.
"But from a moral perspective, it needed to be fixed. I felt there was a mismatch between the mission and the reality and I had to marry those. People were going, `Yer woman's mad, this will fall flat on its face.' "
Staff focused on the transition from primary. They noticed that pupils were losing ground in maths because many were duplicating work they had already done in primary schools. They also recognised that primaries were the experts in mixed-ability teaching.
"One of the big drivers was to get at the brilliant knowledge that is in primary schools," Ms McGuckian says. "They say [Northern Ireland is] the new Finland at primary level. But by the time [pupils] are 15 or 16, they're on their way down to the bottom of the table."
The nine feeder primaries were initially wary. But the schools now work so closely that they routinely take each other's classes at the beginning of the year and use a single scheme of work.
"The fear was that it would be me telling them what to do," says Paddy Hollywood, head of maths at St Patrick's. "But they soon realised that we were learning from them as much as they were from us."
Ms McGuckian says she also faced internal resistance. "One member of staff said, `Some students are never going to make it', that it's cruel to make them try. She's not here any more. Another said, `If one person gets an A in my class, I know I've done my job.' That person is no longer here. The culture was: `We deliver the curriculum, it's up to [students] to get it.' "
In her first year at St Patrick's, Ms McGuckian saw pupils crying as they opened their results. Motivated to take action, she took a group of 29 students who had failed GCSE English but were returning for sixth-form studies and entered them for resits. In November, 25 of the students passed; by the following July, all of them had.
"Even telling pupils that we were putting them in for the higher tier made their chests puff out. They walked taller," Ms McGuckian says.
Another turning point was when she played teachers a recording of her interviewing lower-band students about how isolated and stigmatised they felt. "We were looked down on from Day 1," a pupil told her.
As assistant principal Charles McConville explains, the impact of separating children was deep. "We genuinely didn't realise the depth of hurt that students were feeling," he says. "We did genuinely think we were doing the best for them. We had parents come back to us and say, `I remember being sent to the tech; one of my worst memories was being sent there.' "
For St Patrick's students who now choose to attend Southern Regional College one day a week, the offer has been expanded to 10 subjects ranging from web design to languages and music.
Tackling the gender divide
The school's results used to have a huge gender gap of 20 percentage points between boys and girls. But, having spent 27 years at an all-boys' school, Ms McGuckian felt she could help.
"I had a love of boys: I didn't demonise them," she says. "They don't always fit the profile of the perfect learner. They're challenging, they're boisterous."
Research from the University of Ulster suggests that developing good relationships with boys is key to their engagement. So the principal bonded with male pupils by allowing them to play soccer at breaktimes and cheering them on from the sidelines.
Her approach has led to a rise in boys' results, from 53 per cent gaining five good GCSEs including English and maths to 74 per cent. But girls' grades have also continued to improve, from 73 per cent to 87 per cent.
Ms McGuckian has a personal reason for focusing on teaching students of all abilities: her 20-year-old son has severe learning difficulties. "I was told he's never going to walk or talk," she recalls. "But he does; he's studying at Belfast Metropolitan College, doing a course in independent living. I suppose we're all shaped by our life experiences, and what I've learned from him is never to underestimate human potential."
Read more about comprehensive education in next week's edition of TES
Why St Patrick's High won the award
An impressive turnaround in results meant that, by 2014, St Patrick's had become the highest-performing non-selective school in Northern Ireland.
But it's not all about the academic side: St Patrick's is prominent in the local community, sending food hampers to people in need at Christmas and organising regular visits to the elderly.
As the school's award application points out, the emphasis is on education in its "fullest sense".
In what was a competitive category, the judges said that the "innovative work and move away from banding, the amazing results achieved and rate of acceleration" made St Patrick's stand out.