“Break out the Bolly, Emerald’s just won a bloody Oscar!” I could almost hear the corks popping across the home counties recently when the immensely talented and impeccably posh Emerald Fennell scooped the Academy Award for best original screenplay for her revenge comedy Promising Young Woman. (She was also the first British woman to be nominated for best director.)

Marlborough College, Fennell’s old boarding school, quickly toasted her triumph: “We could not be prouder! Well done Emerald!” I was three years below Fennell at school, and what a PR boon she is for our alma mater.

The 178-year-old co-educational institution in the rolling hills of Wiltshire — or, put another way, just south of Swindon — has otherwise appeared to be an expensive breeding ground for glamorous wives. Boarding costs $56,000 a year.

The extraordinary alumni list includes many alpha consorts: the Duchess of Cambridge, plus her siblings, Pippa and James Middleton; Samantha Cameron and her sister, the Evening Standard editor Emily Sheffield, who was expelled after cannabis was found in her dormitory; Sally Bercow, the wife of the former House of Commons Speaker John Bercow; Frances Osborne, the ex-wife of the former chancellor George Osborne; and Georgina Chapman, the ex-wife of Harvey Weinstein. Jeffrey Epstein’s ex-girlfriend Ghislaine Maxwell is the Old Marlburian who gets glossed over.

Three cheers, then, for Fennell, 35, an actress, writer and director, and her Oscar-winning #MeToo movie. Until her debut film, she was best known for appearing in the BBC’s Call the Midwife and for playing Camilla Parker Bowles in The Crown. “I’m basically playing a chain-smoking posho standing in a corner making cutting remarks. So it’s not a stretch,” she once said of her Netflix role. She has written three novels and was in charge of series two of the BBC’s darkly comic Killing Eve, taking over from her old chum Phoebe Waller-Bridge.

Which is perhaps a good place to pause and check some privilege. Fennell’s parents are the celebrity jeweler Theo Fennell and the novelist Louise Fennell; her younger sister, Coco, is a fashion designer. After Marlborough, she studied English at Oxford, where an acting agent scouted her. But how much did our preposterously privileged school shape Fennell for superstardom?

Marlborough was — and undoubtedly remains — a place of braying yahs, Latin lessons and lacrosse. There was a polo team, a fencing salle and a beagle lodge for hunting. Teachers were “beaks”, sixth-form girls wore bizarre floor-length black skirts and after “prep” (evening homework) pupils snogged behind the school observatory.

She has written three novels and was in charge of series two of the BBC’s darkly comic Killing Eve, taking over from her old chum Phoebe Waller-Bridge.

Many pupils had double-barreled surnames and homes with long drives. Some had famous parents and their 18th birthday parties were photographed for Tatler magazine: Fennell’s Romance-themed party appeared in its Bystander pages. Bryan Ferry’s sons Otis and Merlin went there; Sting’s daughter, Mickey Sumner, was in the same boarding house as Fennell and is also a successful actress.

In the co-ed public school pecking order, it wasn’t the most bohemian (that was Bryanston), or the sportiest (Millfield); it wasn’t full of thickos (Stowe, I’m told) or geniuses (God knows). Rightly or wrongly, it had a reputation for being an all-rounder school for go-getter kids.

There was a strong focus on drama and sport, with Pippa Middleton, during my school years, as the jolly-hockey-sticks pin-up. During one hockey practice I alerted our coach to a loitering “pervert” watching us all. “That’s Eugenie’s bodyguard,” he replied. Prince Andrew’s younger daughter was a year below me.

The writing on the wall at Marlborough College.

In interviews, Fennell, who is expecting a second child with her advertising director husband, Chris Vernon, makes no bones about her head start in life. “I’m always very quick to say a lot of the confidence and ambition that I have comes from an immense place of privilege,” she told Glamour magazine. Looking back, I realize we Marlborough girls had astonishing self-assurance, which teetered into precociousness. Raise your hand, don’t be a wallflower and the world is yours for the taking, was the unspoken ethos.

This survival-of-the-cockiest attitude didn’t suit everyone. “It was a horribly cliquey place where the rugby boys were treated as demigods, the girls had to be pretty or sporty to be acknowledged, and ‘coolness’ was everything. If you didn’t fit that Marlborough mold you were totally disregarded,” said an old Marlborough girl. Most of her friends married Hooray Henrys who couldn’t be accused of overachieving. “I don’t think Marlborough can take any credit for winning Oscars or marrying princes.”

During one hockey practice I alerted our coach to a loitering “pervert” watching us all. “That’s Eugenie’s bodyguard,” he replied.

Besides confidence, our few mutual friends testify that Fennell, who shot Promising Young Woman in 23 days while heavily pregnant, has a ferocious work ethic and inexhaustible energy. Waller-Bridge has called her “a bloody Trojan”.

Unsporty at school, Fennell threw herself into writing and acting. “I found out that you could skive off games if you wrote for the school magazine or had play rehearsals. I had two lines playing Tavern Wench Two in Tess of the d’Urbervilles and never looked back,” she said in an interview in which she also praised our drama teacher, Nigel Bryant. “He treated all of his students like grown-ups and professional actors,” she said.

Others were less positive. My contemporary, the comedian Jack Whitehall, then a class clown nicknamed “Scabby”, staged his own production of Alan Bennett’s Habeas Corpus so he could cast our friends. Whitehall went on to Hollywood but my acting career peaked at 18.

Americans love a posh Brit (I’ve hammed it up as The Sunday Times’s New York correspondent) and Whitehall, 32, and Fennell, must delight the Tinseltown types in Los Angeles, where Fennell now lives. Marlborough does well to focus on charm rather than grades alone, according to Will Orr-Ewing, the founder of Keystone Tutors.

“Marlborough really punches above its weight at helping the kids there to become very socially and emotionally intelligent,” he said. “Because the other way of getting on in the world [rather than just getting top grades] is to be charming and resourceful, so when you’re dropped into an unfamiliar situation, you’ve got the chutzpah to make connections, to make friends, get people onside. Marlborough has got a bit of a superpower in that respect.”

Though when it doesn’t work, he adds, Marlburians can come across as “entitled, lazy, louche”. That sounds about right.

Laura Pullman is the New York correspondent for The Sunday Times of London