"My parents used to tell me not to go into teaching," he says, "but I suspect they knew that one day I would heed the calling."
When he was 19 there was an aborted attempt at a degree. But his passion for maths and its formulae led him into a career in the world of high finance. However, his yearning was to work with large numbers - not earn them.
"I am not motivated by money," says Alastair Cornish. "Which is just as well because teachers have to work long hours for lousy pay. It is not for the faint-hearted. If you want to teach, you've really got to want to do it."
Mr Cornish, a former building society manager and mortgage consultant with a firm of estate agents, finally decided "to go for it" and train as a teacher in 1992.
This meant taking a three-year BA in maths and educational studies, followed by a one-year PGCE at Roehampton Institute.
And now, instead of spending his days arranging mortgages he is, it appears, in seventh heaven, as a newly qualified teacher, busy persuading pupils at Glenthorne High School, Sutton that maths can be enjoyable.
He is convinced his years in industry have given him advantages over his fellow NQTs. Not least his management skills, which he has had to draw on several times over the past months - from organising and managing his lessons to meeting parents outside school.
"This is probably the hardest job I have ever had to do, but it is by far the most enjoyable. It also helps to have exceptional management skills to deal with the stress involved when negotiating with 32 rowdy children," says Mr Cornish.
He does understand why some teachers are stressed out, especially if they are teaching in schools with a large proportion of difficult pupils. He urges diplomacy and advises NQTs not to be afraid to seek help from colleagues. "There are days when I have brilliant lessons with my Year 10s looking at me with 'yes, I understand this' in their eyes instead of 'what are you talking about?'. Those days I think: 'This is why I am in this job.'
"And it is far more satisfying to teach a group of Year 8s who are hungry for what I give them than to sit behind a desk repossessing people's houses."
When it came to deciding whether he wanted to teach in a primary or secondary school Mr Cornish again didn't yield to parental advice.
He says, "With one teaching primary children and the other secondary, they were bound to be biased." Instead, he bore in mind the relationships he was able to build up with secondary-aged children he meets through his involvement with a camping organisation, and a friend's observation that, "If you have a bad class in secondary school it will only be for an hour, and then you can have a good class. But in a primary school if you have a bad class you will have it for a whole year."
But Mr Cornish is under no illusions that the years he hopes to spend teaching will be plain sailing. Already he has had frustrating days when he has felt like screaming at a class of pupils who, at the end of a lesson, still haven't understood the finer points of algebra.
"When this happens I just decide to approach the subject in another way at the next lesson," he says. "You are always living on your nerves and you've got to keep your wits about you if you want to motivate youngsters who would rather not be in a classroom."
He urges NQTs not to have favourites and talks of the preparation necessary to deal with pupils who speak to staff in a way that, if they were employees, they would be shown the door.
He says: "It is important to be fair. Youngsters respect fairness and if you set boundaries they will know how far they can go."