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For Simon Heptonstall, being shot and blown up by a landmine, and watching friends die in battle, has provided him with ammunition to inject large doses of reality into the lives of the young people who surround him at Logan Park High School.
It has been a year since the 44-year-old Yorkshireman resigned from his military duties to come to New Zealand, and these school holidays have been a time for reflection.
He joined the Royal Air Force after he left school in 1985 and went on to become a decorated flight sergeant.
He was part of a regiment which provided ground defence for Royal Air Force installations all around the world, including Northern Ireland, Kuwait, Bosnia, Albania, Sierra Leone, Afghanistan and Iraq.
During his tours of duty, he's been shot twice.
He recalled how lucky he had been when he was shot from behind by an enemy soldier with an AK47.
''I had a radio strapped to my back, and the bullet went through it, punctured my body armour and went into my back.''
Without the radio and body armour, he would almost definitely have been killed in action.
He also had a piece of shrapnel lodged in his arm after one of his colleagues stepped on a landmine. Sadly, his colleague didn't make it, he said.
''I was 18 when I saw my first deaths. After that, you just get used to it - or do you really get used to it? I guess it becomes a job.
''If you get too emotionally attached, it can mess you up.
''It's hard not to get attached, but in the end, you've got a job to do.
''I was living in the line of fire. I've got a few scars.''
Mr Heptonstall said he had lost count of the number of friends he had lost over the years.
''Due to the job, too many of my friends aren't here anymore and I didn't want to think of my family suffering like their families - without a dad or a husband.
''I've got a wife and a 3-year-old daughter.
''I've known my wife since we were 7 years old - we were inseparable. She's my childhood sweetheart.''
So in December 2011, he resigned and less than a month later, he had moved with his family to Dunedin, where he is now director of the Logan Park High School Service Academy.
He describes it as a ''military-style'' alternative education programme for 16- to 18-year-old youths at risk.
''This is a great opportunity for me to bring all the skills I've learnt from my career and share them with young people in Dunedin.
''All I'm doing is showing them boundaries and what's right and what's wrong in life.
''I'm their dad, their mate, their brother - whatever they need me to be.''
Finally, it seems the fruits of his labour behind enemy lines are being realised.
''I find this job unbelievably more rewarding than my old one,'' he said.