Life in company boardrooms can be pretty bloody. Directors and chairmen enjoy high status, but they play for high stakes. A thick skin and a steady nerve are prerequisites for steering businesses through the turbulence of the marketplace.

Nobody knows this better than Andrew Garety, who at the age of 53 has navigated just about every peak and trough imaginable at some of the UK’s best-known companies. He was finance director at brewery firms Morland and Boddingtons; chairman of wallpaper group Vymura; and, most notably, finance director and then chairman at Liberty in the 1990s, where he found himself in the thick of boardroom walk-outs and sackings during a particularly well-publicised and battle-strewn period of the department store’s history.

Mr Garety has relished the cut and thrust of business life but at the very pinnacle of his career, when he could easily have opted for a comfortable portfolio of non-executive directorships, he made a move that sees him playing for stakes of a different kind.

Last September, Mr Garety moved out of the boardroom and into the classroom to undertake postgraduate teacher training at Manchester University. After the summer, he will take up a post as an English teacher at Alleyne’s high, a 13-18 secondary in Staffordshire. “I was at that point in my life when a couple of jobs had come to an end and I had been approached for non-executive posts, and I suddenly thought, ‘I’m not sure I want to do this any more’,” he says. “I’d enjoyed a successful career as a finance guy and seen just about every competitive activity there is to see. Going into teaching suddenly seemed the obvious alternative. Way, way back, teaching was something I had wanted to do.”

Mr Garety does not underestimate the challenges, but is unequivocal about the job satisfaction. “In teaching, the rewards are more immediate,” he says. “There is nothing quite like the satisfaction of teaching a lesson you know has gone well and where the kids have been excited and learned a lot. In business, the process is slower burning before you see the results for shareholders and employees. But the lows in teaching are more immediate as well - for example, when a lesson has not gone well. I’m not so used to that.”

Though a man who has obviously thrived under pressure, Mr Garety was unprepared for the demands placed upon him as a trainee teacher. Since September, he says, he has taken only five weekends and one week off. “I knew teachers worked jolly hard, but I didn’t know quite how hard and how physically demanding it is,” he says. “You are at the beck and call of so many people; you never stop during the day. In terms of work, I would say it is the most open-ended profession - there is always more you can do. Next year, when I have a full teaching load, it will be even harder. But that’s not going to stop me. Potentially, you are in a position as a teacher for creating an enormous amount of good.”

What drives him, he says, is a simple desire to help children learn. “If you can get even a handful of kids to develop their potential, particularly if they have been underperforming, it’s worth it.” He says a fascination with the machinery of business rather than a love of status or financial reward has driven him through his career. Business colleagues were “shocked” but “not surprised” at his decision.

Nor is it the first time he has made a complete change of direction. As a school leaver he had been accepted to read English at Cambridge but decided at the 11th hour that he wanted to become a doctor and persuaded the university to allow him to switch to medical science even though he had no science A-levels. His new interest had been fired in the sixth form when he had read John Berger’s A Fortunate Man, the biography of a country doctor.

But two years into medicine he became disillusioned with its “very academic, non-vocational” approach and switched back to English. He left Cambridge with an upper second degree, despite having completed the subject in a year, and stepped straight into a graduate traineeship at Boots; a few years later he qualified as a chartered accountant with Price Waterhouse. He relishes the prospect of completing his working life in the classroom, though school management does not attract him. “I’ve done that. I start with the clear position that I want to stay in the classroom.”

Changing careers at the top of a professional ladder can give people a new lease of life, according to Cary Cooper, an organisational psychologist based at the University of Manchester Institute of Science and Technology. Professor Cooper says people who make the change to teaching in their 40s and 50s, having been highly successful in another walk of life, are reinvigorated by the prospect of developing young minds. “Yes, they will have to work long hours, but they are stimulated by the challenge of making a contribution to society in this way and so they will be fresh; they will have a new energy,” he says. “Most people who have been successful in a job for 15 to 25 years are burned out and need something different.”

The proportion of over-35s doing postgraduate teaching courses rose from 18 per cent to 18.9 per cent in 1999, the last year for which government figures are available, while the number of PGCE students aged over 45 remained static at around 3.6 per cent. Recruitment expert Professor John Howson says that because 20 per cent of mature entrants don’t get jobs, teacher training colleges tend to favour those whose prospects are better. “It’s easier to get in if you’re a high-flying business executive who might be a good bet for a secondary school,” he says. “You will look more attractive than a 35-year-old nondescript person wanting to transfer.”

Lizzie Ryan, 31, a former City trader with equities firm DLJ, is undertaking teacher training at the University of Greenwich, and will become a maths teacher at Sedgehill, an 1,800-strong multi-ethnic secondary in the London borough of Lewisham, in September. Like Mr Garety, she was successful in finance and had thrived in the laddish culture of the trading room, but wanted a job with more social responsibility.

Ms Ryan left school at 16, gaining qualifications and a degree only in her late twenties by studying part-time as she worked her way up the City ladder. That required long days, rising at 5.30am for a 6.30 start, and entertaining clients or studying late into the evening.

Teaching, she says, is equally demanding, though very different. “You have to be on the ball, on show all the time. You can’t slope off into a corner if you’re feeling bad. People are here before 8am, they grab a sandwich for lunch, they stay late, they never stop. But there’s that feeling of responsibility. You are dealing with individuals, with fragile beings, and that is the hardest challenge. You have extreme highs and extreme lows. Sometimes, after a bad day, you just want to go home and cry, but if you are making a difference, if you are reaching out to even a few, it is worth it.”

Nor is she put off by the increased administration, which she believes is necessary to acquire essential information. “There have to be records. It’s like any business - you have to have information to be effective. In the City, we never implemented change without seeing the bottom line,” she says.

Ms Ryan believes her own experience of school helps her relate to her students. “I think it helps me understand difficult kids, to have more empathy with those who are bored or can’t be bothered. I was like that. Now I think, ‘What would have made a difference to me at 14?’, and I try to work towards that.”