For more than four decades, Peter Thomson was a flamboyant, tartan-trousered presence in London’s independent schools.

His defining moment, however, came during the Clapham rail disaster of 1988, when he rallied staff and pupils to rescue passengers trapped by the collision of three trains at the edge of his school field.

Peter Findlay Thomson was born into an Edinburgh military family in January 1939. While he was a pupil at Haileybury School in Hertfordshire, his alcoholic father left home; Peter did not see him again.

Growing up without a father, the teenage Peter felt a deep desire to help others in a similar position - to serve as a surrogate parent to those in need. And so, after receiving a first-class degree from Trinity College, Cambridge, he became a teacher.

As history and divinity master at St Paul’s, the London independent school, he exuded heartiness and intellectual passion: one former pupil recalled a man “even grander than the headmaster”. He made a point of knowing every boy, rewarding those who worked hard with extra help and guidance.

But he also had a soft spot for quirkiness: for pupils who overcame the academic odds, or for depressed boys who needed someone to listen to them. On discovering - to his horror - that one quiet boy neither played sport nor knew Latin, he made a point of regularly greeting him with a pithy Latin remark about health or fitness.

He left St Paul’s briefly to work as housemaster at Radley College, in Oxfordshire. But he returned two years later, this time as surmaster, St Paul’s version of deputy head.

He had married in 1962, and raised a son and a daughter. When the marriage ended, he found himself sharing the school run with Sheila Holmes, a neighbour with a similarly aged daughter. They married in 1984.

That same year, he was appointed headmaster of Emanuel School, in south London. According to the chair of governors, he “took the place by storm”, increasing pupil numbers by more than 25 per cent.

The defining moment of his headship, however, came in 1988. On a December morning, just as pupils were arriving at school, three commuter trains collided half a mile south of Clapham Junction, just beyond Emanuel’s sports fields.

With no hesitation, Mr Thomson sprang into action. He immediately gathered sixth-formers, rugby-players and any other boys strong enough to help. Before official rescue workers had arrived on the scene, Emanuel pupils were pulling survivors up the embankment, taking them to safety, making them cups of tea.

He refused to countenance the possibility of sending boys home afterwards: school must carry on as usual, he insisted. And so, on the same day that its pupils pulled injured commuters from train wreckage, Emanuel held its annual carol service.

He firmly believed that no headteacher should remain in post for more than 10 years, and therefore he retired in 1994. This retirement, however, was short-lived: after a brief spell as interim head elsewhere, he was persuaded to take on the headship of south London’s Harrodian preparatory in 1996.

This return to the staffroom afforded him a new lease of life. Naturally flamboyant, he strode the corridors in panama hat and linen suit in summer, tartan trousers on special occasions. When a Harrodian pupil was accepted to Haileybury, he donned his old school blazer and presented the boy with the socks he wore during his time in the Haileybury first XV.

The emphasis at Harrodian was on responsibility rather than results. Nonetheless, in his first year as head, Mr Thomson oversaw a 100 per cent pass rate in the common entrance exam.

Above all, however, he enjoyed monitoring the dinner queue: every day, he would talk to pupils about their work, their lessons, their lives. He never missed a football or rugby match, and each year travelled to watch drama groups perform at the Edinburgh festival.

He was similarly loyal to Fulham FC. In 40 years as a supporter, he did not miss a home game, and wrote and self-published several books recounting his life of fandom.

In 1999, he became school principal, taking on a more statesmanlike role. He was, however, instrumental in expanding Harrodian through to GCSE, in 2000, and to sixth-form three years later. This allowed him to offer history tutorials to Oxbridge candidates, which he relished.

He continued to teach until he was admitted to hospital in late April. Alert to the last, he was in the middle of a conversation when he died on 5 May.