There cannot have been many resignation letters as unusual as Paul Winter's.
The special school head has left a 30-year career in education to become a professional athlete - and has already been picked to represent Great Britain.
Mr Winter, 52, from Rottingdean in East Sussex, has discovered late in life that he is one of the world's best triathlon athletes in his age group (50-55).
The head of Downs Link College in Brighton said: "It has all come around quite quickly. I started doing triathlons last year and people kept telling me I was good for my age. Then all of a sudden I've become the national number one and I'm picked to compete in major championships.
"It's a rare opportunity for anyone ... I'm reluctant to leave (the school) but I can't pass this up."
Mr Winter took up triathlon last year after looking for a new sport, motivated by the recent death of his wife, Lesley, from leukaemia.
He had been a competitive sailor, racing at a high level, until a back injury forced him to quit in 1997.
"I've always kept fit but I'd never run or cycled competitively and hadn't swum in competition since I was a youngster," he said. "I didn't really know what to expect when I did my first triathlon, but I finished second. Then I finished second in my next race, and third after that. And then I started winning."
Buoyed by his growing success and encouraged by friends, Mr Winter entered the national ranking series of races. At the end of the season, he had been placed as number one in his age group, qualifying him for major international events.
Mr Winter will leave his job at Easter to train in Florida, where he will prepare for next year's European Championships in Holland and the World Championships in Australia. "I'll probably only have about two years competing at international level, so maybe I'll go back to teaching one day." he said. "You never say never."
Peter Holmes, a spokesman for the British Triathlon Federation, said: "What Paul has achieved has been fantastic. He's shown himself to be an impressive competitor after such a short time. We look forward to seeing him achieve some more success on the international stage."
Adrian Carver, head of Downs View Special School, where Mr Winter used to work, said: "We will miss Paul because he has been an excellent professional here with a huge amount of passion and commitment.
"It is that same passion and commitment that has helped him to become an international athlete. We wish him the best of luck, and hopefully he'll come back and show us his medals next year."