As careers go, they are about as far apart as it is possible to get. One is dedicated to making money, with the extravagance of the rewards matched only by the job insecurity; the other is devoted to the common good, with thoughts of material gain subordinate to public service.

But these are the two worlds spanned by Alex Brassey, who traded in his career as a City high-flyer - at one point handling pound;1.5 billion a day in the foreign exchange markets - for life in the classroom, teaching A-level pupils about currency movements.

“I was involved in investment banking for a long time, but it got to the point where I saw people who were earning millions and they were feeling quite unhappy,” says Alex, 34. “It took me a few years, but I decided I’d had enough and life was about more than money.”

After taking a PGCE at the Institute of Education in London, he now teaches business and economics at Copthall School, a girls’ comprehensive in Barnet, north London. There was little in his earlier career to suggest he would be attracted to teaching.

Alex began trading in shares when he was 12, using money his grandmother had set aside for him. Starting with British Telecom, he invested in almost every privatisation going. “I was one of Thatcher’s children,” he says. He moved from privatisations to the foreign exchange markets, and while at the University in Manchester he had a Reuters screen installed in his bedroom in Liverpool so he could continue trading. While his peers were holidaying in youth hostels, Alex was staying in five star hotels.

His track record meant he walked into a job in investment banking, where he became a currency trader, working in a market that handled pound;600,000 million a day. It was a high-pressure environment: brutal to those who fail, but generous to those who excel. Alex rose swiftly.

He became one of the world’s leading dollar-yen dealers. At 24, he was advising governments; on some days, his decisions would move the dollar half a per cent. It was thrilling, and fulfilling, and a little frightening.

“It was bizarre to think you could change the value of America. I often felt like a cartoon character running off a cliff and pedalling in mid-air, and then you look down and you drop. You feel you shouldn’t be doing what you’re doing - and you can’t think about it too much.

“The stress was constant and you’ve got to be incredibly on top of things.

You’re being observed all day every day, and your decisions go on the screen immediately, so you are constantly accountable.”

He’s coy about revealing how much he earned - high six figures is as much as he will say - but insists it’s a myth that everyone in the City is handsomely rewarded. He also saved most of his money, and when he did indulge in the lifestyle associated with City traders, it was usually at his clients’ expense. He tells one story about going to a fashionable restaurant and ordering a bottle of wine that had been name-checked in the Len Deighton thriller he was reading, which came in at pound;1,200. The second bottle was poured to nods of appreciation around the table, until the wine waiter pointed out it was corked.

“Everyone was always trying to outdo each other. I was taken out to every posh restaurant in London and you would have seriously expensive wines, but really I’m more of a Pizza Express man.”

He married Vanessa, and it was around the time his first child was born, in 2000, that disillusion seriously set in.

“It’s a world that is solely about money: there is no ethic, no human value, no interest in anything apart from making money. I’m not passing a value judgment, although I suppose I am because it is disgusting.”

After taking a redundancy package, he did a master’s degree in philosophy, and then enrolled on a PGCE course. “I didn’t have to worry about money, so I looked for a career that was compatible with a good work-life balance and where I could contribute something from my previous experience. Teaching is a much nicer environment and the pressure simply isn’t the same.

“The end product has nothing to do with money and you are operating in a different environment. The outcome is educating children and that is significantly more rewarding.” He’s been at Copthall since September 2005, and hasn’t been afraid to exploit his mine of stories to explain how currency markets work, and his contacts, arranging an annual trip for his sixth formers to the floor of the London Metal Exchange. But his experience has convinced him that schools do not make enough of the skills brought in by teachers who had a previous career.

“It’s high time the education system opens itself up and takes on people from a wider background, people who’ve had experience at middle and senior management levels,” he says.

“One of the reasons it isn’t happening is because teachers insist that to progress up the ranks you have got to have been through the system, but by the time anybody becomes a head they will have forgotten everything they learned in the commercial sector.

“There is a fear of change and it is frustrating. Often teachers say to me that it is education, not the commercial sector. I appreciate that, but it doesn’t mean education can’t do things better.”

He has been able to put his skills to good use, however, in organising a fashion show for London schools, which takes place next Thursday at the Arts Depot in Finchley. His fellow teachers also regularly ask him for investment advice.

It’s probably Premiership footballers who come closest to City traders in the extravagance of their lifestyles, and like many former footballers, Alex is happy to hark back to his glory days, although he insists there is no going back.

“There are moments when I miss the excitement, the adrenalin, and the uncertainty, but there is a downside as well. When it’s going right, you can make the entire market move because of your screaming, but there are days when it can go horribly wrong and you wouldn’t know whether you would have a job. There is a part of me that says I would go back, but I know I won’t.”

Make the switch

An increasing number of people are entering teaching after switching from another career. The proportion of people entering initial teacher training who are over 30 has risen from around 29 per cent in 199899 to 32 per cent in 200405, the latest year for which figures are available.

A study carried out by the Training and Development Agency for Schools (TDA) published in January shows one in five newly qualified teachers gave up a management or senior role to turn to teaching, with another 16 per cent leaving a middle management position.

Making a difference to people’s lives was cited by 79 per cent as a reason for switching from another career to teaching, while 78 per cent said they wanted to work with young people and 75 per cent said they were ready for a new challenge.

Details of taster courses in England and Wales - three-day courses, including a one-day school placement, for people thinking about teaching - are available from the TDA website, www.tda.gov.uk.