When Anthony Seldon, master of Wellington College, told his pupils last week that he was planning to leave, he said he was weighing up three options for his next career move: taking over as manager of Manchester United, becoming the next 007 or being the 45th president of the US.

He was only joking, of course. But somehow, you feel that Dr Seldon, an indefatigable school leader with as many fingers as there are pies to put them in, has probably thought of becoming all three.

The outspoken educationalist, historian and author - the closest Britain has to a celebrity headteacher - will leave his prestigious private school in Berkshire in the summer of 2015 after nine years at the top. A “fresh figure” should take over, he said. And he needs time to spend with his wife, Joanna, who is seriously ill with cancer.

Dr Seldon, who practises yoga and meditation twice daily, is probably most popularly known for introducing “happiness lessons” to the curriculum in 2006. The lessons in well-being were ridiculed as “psychobabble” in some quarters, but have set a growing trend.

He has also become a leading advocate of partnerships between private and state schools, writing and speaking about the topic extensively. His fervour and idealism have on occasion been mocked by other private sector headteachers who have half his energy and far smaller budgets. But in 2009, Wellington risked its reputation by becoming the first private school to act as the full sponsor of a state-funded academy - the Wellington Academy in Wiltshire.

The move appeared to go well at first, but there was more than a hint of Schadenfreude from opponents of the private sector when the school’s GCSE results were found wanting last summer, which led to the departure of its headteacher.

It has been a frustrating time for Dr Seldon, but he is always keen to look on the positive side.

“I’ve learned more since September about schools and teaching and leadership than I have in several years in the independent sector,” he says. “Not since I did a PGCE at King’s [College] London, 30 years ago exactly, have I had such a masterclass in teaching, learning, leadership, monitoring and assessment, and behaviour for learning.”

Dr Seldon says he feels an “overwhelming sense of gratitude” about his time at Wellington College. “It’s been the best experience of my life. I feel I have learned so much from it,” he tells TES. “It’s a hard school to run - it’s a very big school and very high-profile and there were all kinds of media issues when I arrived.but it’s all been an utterly remarkable experience.

“I know it’s right [to leave] and I’ve got to do something different. The next person will come along and be much better.”

Dr Seldon says he has gained much from his time at Wellington, gleaning insight from other teachers through his involvement in the G20 global group of schools. There has also been considerable international expansion, with Wellington outposts established in Tianjin and Shanghai, China. A third school is already in the pipeline.

And, finally, there are the books: he has written at least 10 while running Wellington. One of his most recent offerings, Public Schools and the Great War, examined the role of private school pupils in the First World War. Well-known for his biographies of prime ministers, including Tony Blair, Dr Seldon is currently writing one about David Cameron’s time at No 10.

So, with this frantic period nearing a close, does he have plans to take it easy in his final year?

This is unlikely. “You can judge someone’s quality as a professional from what they do from the moment they announce they are leaving,” he says. “Some people just sort of give up and let go. I want to be working as hard on my last day here [as I ever have].”

But the best times for Wellington College lie ahead, he insists. “If you work hard, the glory days of a school are after you leave.”