My parents looked at all the day schools that were on offer - it was tacitly assumed that they would have to be Protestant ones since education was, in effect, totally segregated. After talking to lots of people they chose the High School. Academically it was clearly the pre-eminent school in Dublin - despite its primitive resources.

It has now moved into splendid modern premises, but in the early 1950s the school was based in an old Georgian house about half a mile from the centre of the city. It had expanded over the years into the servants’ quarters at the top of the house, and, by the time I went there, many classes were taking place in the old stable block.

The headmaster taught a Greek class in a bathroom, which still had a huge bath in the corner. It didn’t bother him and it didn’t bother us. We still got perfectly good results - something which makes me feel that nowadays there is perhaps too much concentration on the need for more resources rather than on the quality of teaching.

There were other peculiarities. The caretaker wasn’t well-paid, so he was permitted to raise a pig behind the handball courts. And the school was organised in a way that would now be considered very politically incorrect: the classes were re-arranged every term on the basis of examinations held at the end of the previous term - you could always tell who was the dunce because he was sitting at the bottom of the class.

Being an English boy in an Irish school gave rise to interesting possibilities - which included being cast as King James II, fleeing after the Battle of the Boyne, in an Irish-language play (Irish being a compulsory subject) written by Eamon de Valera’s wife. She was present at the performance and I spent most of the time being rolled around the stage in a barrel. The reviewers in the Irish papers were delighted with the symbolism.

In a way, it’s invidious to single out one teacher from a very talented group at the High School, but my maths teacher, Victor Graham, was by far the most influential. He was in his early forties and was also a maths lecturer in the engineering department at Trinity College, the university down the road. Every afternoon he came to the school to teach the senior classes who were studying for matriculation or for scholarships.

Victor Graham already had a long line of achievements to his credit by the time he taught me - a whole succession of people from the school who’d gone on to take up academic careers in mathematics. He was not dismissive of those who didn’t have an aptitude for maths, but what was remarkable about him was the single-minded way he helped the ablest sixth-form students. He did that not only with his classes in school but by his invitation - which was close to a command - to attend on him every Saturday evening at his home in the suburb of Rathgar.

Victor Graham lived there with his parents, and he would dedicate three hours of his Saturday evening to teaching promising pupils to a higher level than he could teach them in the class. He had an impressive maths library which he kept up to date by getting all the journals he could find from around the world.

He was unmarried (legend had it that he had been jilted) and his whole world seemed to be mathematics - and music (he was a good pianist). He was a slight, dapper man who wouldn’t stand out in a crowd, but when you engaged with him at a personal level he had a tremendously strong personality. Younger pupils whom he didn’t teach sometimes found him intimidating, but those in his maths coterie formed a very strong affection for him.

Anyone who was taught by him learned method, an understanding of the need for rigour in mathematical reasoning, and, in particular, tenacity in trying to solve problems - for days, if necessary - that didn’t yield an easy solution. If that sounds a bit prosaic, he was also able to convey a sense of the form of mathematics, of the elegance it has at its best, of the attractions of mathematical reasoning.

Even if in later life I haven’t needed advanced mathematics very often, I have found the reasoning approach which Victor Graham inculcated in us very helpful in the analysis of problems. I was lucky enough to have a natural aptitude for the subject, but he set me on the path which gave me a platform for higher ambitions - and probably helped me to get into the Harvard Business School, for example.

Victor Graham died some years ago - relatively young and still teaching - and many people contributed generously to a memorial fund. It was widely recognised that he had been a remarkable man and an exceptional teacher. When mathematics is poorly taught even quite promising people don’t fulfil their potential. Victor Graham’s greatest pleasure came from putting young people on the right track in the right way - he was evangelical about that - in a subject that he loved.

David Challen is chairman of J Henry Schroder, the London investment bank which advises companies on mergers and acquisitions. After graduating with a gold medal in mathematics from Trinity College, Dublin, he started his business career with the J Walter Thompson advertising agency, before joining Schroder in 1972. He is now a non-executive director of Anglian Water, has served on the Takeover Panel and is a governor of Morley College, London