Mr Booth, an ex-rugby player and former warrant officer in the Australian army, cuts an imposing figure through the school halls as master of discipline.

The job is the only one of its kind in Canberra, a relic from the school's early days running cadet programs, and one of just a handful around the country which typically see ex-soldiers and police in charge of maintaining "the tone" of their school.

But students and staff say Mr Booth’s work goes far beyond straightening ties - and the “big man with a big heart” has changed the lives of countless students under his care.

He coaches rugby, runs the weight room, and helps out with the school's dragonboating squad. He teaches kids to be tough, both on the field and in their own heads, says Year 12 student Matthew Thornton.

Stories of ‘the Sarge’ reached Sarah Lorenz long before she came to the school in its first wave of co-educational enrolments last year.

“I remember thinking whoever's in this role must be this really scary, mean person...I was really nervous [to meet him] and I was one of the first girls at the school to get detention,” Sarah says.

“But then he was so kind about it, he put me in charge of the younger kids. He can be stern but he’s so humble, it makes you feel safe having him here as sergeant, he sees the good in people.”

That includes students who've "gotten into mischief", according to school captain Nick Dimoff.

"He's our mentor, he's always on our side, he just wants us to be better," he says.

When Mr Booth took the job straight out of the army, he only knew the previous sergeant, an alumnus, had passed away. But within days, students were lining up outside his door, and parents ringing to praise him. The work was remarkably like his time training cadets at army bases, and along with his wife Debbie, it wasn't long before he was stepping in to help students in need.

"It might just be a tap on the shoulder, Mum might be away in the army or dad's away working to pay the fees because not every kid's parents are loaded that come here," he says

"You can guide the kid without [trying] to be their father."

One student he had a particular hand in training on the rugby field was Waratahs lock Tom Staniforth, who graduated from Canberra Grammar in 2012. Even at 5:30am, Staniforth says the Sarge would be at school ready to let him into the gym or an empty classroom to study.

"That's how I'd start my mornings," he says, recalling 'phenomenal" fitness sessions every Tuesday with the Sarge running military-style drills.

"He'd really go for it, he always looked after ya, he looked after everyone, it didn't matter who you were or what you did, I'm pretty sad to hear he's leaving but wherever he goes he'll have an impact."

In his fourteen years at the school, the Sarge has never once taken a sick day. But this week, his final one at Canberra Grammar before starting a new chapter at The King's School for boys in Sydney, he admits he considered making the call.

“I’ve never been able to do goodbyes,” he says. "Every year the year 12s will come looking for me but I have a real phobia, it goes back to my childhood and [a family] breakdown, but I felt I owed it to the school this time.

"[Resigning] was the scariest thing I've ever done in my life...I said to my friend from [the army]...I'd rather be out with you running operations."

More than a thousand people packed out the school this week for his final farewell assembly, including at least 20 'Old Boys' school alumni.

"It was pretty incredible when we all came together and clapped him out," Nick says.

Those that had wondered how the Sarge would take to a co-ed environment didn't know him, Canberra Grammar head Dr Justin Garrick says.

"Despite his imposing demeanour...there will be very few students who do not recognise his fundamental kindness and his care for their wellbeing, no matter how firmly that care might have needed to be expressed."

While the Sarge admits he gave the girls a three-month "honeymoon period" to settle in, he'd had no problems.

Sarah, who was once chronically late to school, even started getting up earlier in the mornings.

"I respect him so much, I didn't want to let him down," she says.

Mr Booth, who survived bullying during his own "difficult" childhood, says there had only been five kids at the school who shut him out.

"But of those five, three of them came back to see me years later...I try to make a connection with all them, I know how hard it can be."

Moving schools every two years to follow his public servant father around northern Australia, he says he never quite fit in.

"My mother was Maori but dad was white so I had some schools where I was the only black kid and other schools where I was the only white kid."

"But I''ve woken up every day for the last 14 years and wanted to come to school. Now, I want to take the sergaent model [to King's] and see if it works."

Canberra Grammar will look to fill his "big shoes" next term, but, while the job title will likely remain 'sergeant', a spokesman said the person might be very different, not necessarily from the military.

"The crucial thing is to find the right balance of personality; someone who brings the same integrity."