When Anthony Seldon left Brighton College, where he had been head for eight years, it was obvious it wouldn’t be the last we heard of the self-styled maverick educator.

Renowned for dramatically improving the school’s fortunes (when he arrived it was in £3m-worth of debt and underperforming academically), he was equally well known for unconventional behaviour, such as sitting an A-level alongside pupils and competing in sport days.

He was often to be found sounding off on issues of the day (frequently in the pages of The Argus) and was a fierce critic of the city council, whose leadership he found lacking in imagination and verve (he is still riled that no one took up his suggestion of a counterpart Royal Pavilion in Hove – “Chinese on the outside, Indian on the inside”.) True to form, he’s been no more retiring in his current position at Wellington College in Berkshire, where he has made headlines by introducing happiness classes and, recently, by putting his belief in the potential of state-private links to the test at Wellington Academy, a comprehensive the independent Wellington College has sponsored for the past four years.

A busy man, then. “Oh no,” he says gaily, on the phone from his office at Wellington. “Don’t believe what you hear. I’m just doing my nails.”

But his jolly tone masks a rather more anxious mood, especially when it comes to his work at Wellington Academy, whose head Andy Schofield (formerly of Brighton’s Varndean School) stepped down in September after poor GCSE results. Seldon has since stepped in as executive head.

But while he had always harboured ambitions to work in the state sector, he says he was naive about what it actually entailed. He was reported to have been seen “screaming” at pupils during one assembly this term, with witnesses claiming he was “going bonkers, telling them they were the most badly behaved kids”.

It’s a thorny subject, he mumbles, not least because parents at Wellington College, who pay fees of up to £32,940 a year, are “sensitive” about the time he’s spending at the Academy. “It’s difficult to talk about this until I’ve got the school sorted out but yes, I was certainly naive.”

He had tried to open an academy while at Brighton College and had even secured a sponsor willing to put up £2m towards it but again, was thwarted by the council.

“I don’t know why but they just wouldn’t have it.” It was a disappointment that surely contributed towards the disillusionment he describes during his final months at Brighton. Despite his many successes at the college, he says he found his tenure “quite a gloomy period in my life. I felt quite weighed down after I left.” He had long suffered from a naturally pessimistic disposition and to combat these feelings, he rediscovered yoga and meditation, which he had last practised while at Oxford University. He also discovered American psychologist and self-help guru Martin Selligman and his theories on positive psychology.

Selligman’s messages – that we should focus not on what’s wrong but what’s right and that we can learn to be happy – came as a “hell of a thing” to Seldon and not long after his discovery he co-founded Action For Happiness, which describes itself as a movement for positive social change.

He has since introduced happiness classes to Wellington College. “We do a lot on growth mindset, helping people realise they can change their lives.” Now he says his default position is positive, although I’m not sure I fully believe him.

He argues that it is our duty to at least try to be happy, for people around us as much as ourselves. His own upbringing was lived in the long shadow cast by his grandfather Wilfred, a First World War survivor whose suffering in the trenches left him prone to violent mood swings. He would go into rages that made his children miserable and anxious, tendencies they inadvertently passed on to their own children.

“The damage we can do to people can go on for a long time,” remarks Seldon, “If you’re brought up to think the world is a fearful place, it’s hard to break out of that brooding mistrust of other people and what might happen.”

His fascination with the First World War seems in part an attempt to dissect his own family history. His latest book (he’s written dozens, most famously political biographies on Tony Blair and Gordon Brown) was inspired by his grandfather’s experiences in the trenches. As a junior officer in the London Rifle Brigade, Wilfred was in Ypres when his valet was shot in No Man’s Land in December, 1913. When Wilfred clambered out of the trench to tend to him, a German sniper shot him in the head. Though he survived, he was never the same again. “It mucked up his life and it mucked up my parents’ life,” says Seldon.

His book aims to challenge the popular conception that it was the public school ethos that caused needless suffering on the Western Front or, as Seldon puts it, to counter the Blackadder stereotype of privileged, cowardly buffoons sending men recklessly to their deaths. He argues that the public school education of officers such as his grandfather (a St Paul’s pupil) imbued them with an idealism and stoicism that helped them care selflessly for their men in dire conditions.

“They were incredibly self-sacrificing to the extent that they were dying at twice the rate of ordinary soldiers,” he says. “I’m not saying those from public schools were the only good soldiers or that they were always heroic. Some of them were cowards, some of them were snobs, some of them were bullies. But I am saying these people were extraordinarily heroic in what they did and felt and saw and the way they inspired their men.”

But he insists the book is not a defence of public schools. “That’s not me really. Have they been maligned? Of course they have. They have been blamed for a whole set of attitudes that led to this massive suffering. I’m not saying love them, I just want to set the record straight.”

And what of his own place in the history books, I wonder? What legacy does he hope to leave? Here, positive thinking fails him. “I haven’t achieved a thing,” he says, sounding thoroughly gloomy. “Almost nothing I wrote about in Brave New City [his 2002 vision for Brighton] has come about. The Brighton Academy hasn’t come about. All I can say is I’ve tried to do my best. I have lots of time left and I hope what I do now is going to be better.”

  • Public Schools And The Great War by Anthony Seldon and David Walsh is out now (Pen And Sword, £25)