You stride across the Fiesta-filled car park to the Staff Only door. Then you’re in: officially on the payroll, a newly-qualified teacher. Slightly woozy, smiling too openly, you take a mental picture of yourself as Sir in a suit you bought yesterday. This, you tell yourself, will never happen again.

Two years later, it happens again.

This time you’re in a parking lot and the Fords are much, much bigger. But it’s six months since you moved to Ontario, so you’re getting used to these chrome-dipped, Detroit-sourced sedans. What you’re less comfortable with is your new job title - a “beginning teacher”.

The regression started in the autumn. You learnt that if you wanted to teach in Canada’s most populated province, you’d have to be accredited by the Ontario College of Teachers, the profession’s self-regulating body. After months of form-filling, you persuade Toronto’s bureaucrats that you aren’t “unqualified or unfit” to force your strange accent on to Canadian kids. You are free to start teaching.

So, take two. But your mental picture seems distorted: you’re not in a suit (no ties allowed) and the Staff Door is the only way in. You’re new students don’t need an entrance as they’re not allowed out . . . for years, usually.

You’re thinking about that when a formidable, blue-shirted woman with fat, rubber-gloved fingers beckons you towards her desk and asks for your driver’s licence. Then, satisfied with the photo, she rummages through your bag, sees what you have for lunch, and wishes you a nice day.

In jail.

A metal gate slams behind you.

Until now, teaching in prisons - what Canadians call correctional services - was just an idea and the eight high-walled prisons scattered around the small, tourist-friendly city of Kingston were just places you passed on the way home from the harbour or Ye Olde Ice Cream Shoppe. Some, such as the architecturally dramatic Collins Bay Institute, look more like Disneyland palaces than panopticons.

But not from the inside.

Shuffling along hot, dark corridors, you wonder what you are doing here. If you weren’t so securely locked in, you’d take your transferable skills and run. It’s only when you cross Barrier 34 that things become familiar. You recognise the fluorescent glare, the dusty shelves, the piles of books, the rows of desks. It’s the universal classroom.

The students look different, though. Yes, they’re slouched in ergonomically disastrous little chairs with textbooks and uniforms, but these are big men. You wouldn’t want to tell them to sit up straight or stop arguing.

And you don’t have to. Here, behaviour management is a personal alarm you wear on your belt. At the first sign of trouble you press the button and walk straight out of the classroom. Intervention strategies are left to the guards and their hand-cuffs.

So you’re free to teach. It’s rewarding, too.

Students work on their own projects at their own pace, so teachers act as facilitators - circulating, helping out, setting work. The atmosphere is surprisingly relaxed, surprisingly humorous. You ask one inmate whether he’ll get his work finished. “I should do,” he replies. “I’ve got 12 years in here.” When another boasts about how smart he is, the room erupts with laughter. “Sure,” says his tablemate. “That’s why you’re in jail!” It isn’t always funny. Tony, in his early twenties, tells you how angry he is with the system that ignored his disruptive behaviour in high school. Why, he asks, is he only getting help now that he is locked up? Although he takes the blame for his crimes, he wonders what his life would be like had there been more accessible counsellors and psychologists earlier.

You’re about to tell him that you know how he feels. But that would be crass; you don’t have the faintest idea. So you tell him that people might be reluctant to pay for the extra school support he needed as a child.

What kind of answer is that?

Nicholas Woolley taught English for two years in a Manchester comprehensive. He now lives in Kingston, Ontario, Canada, where he teaches inmates in correctional institutions