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Bo Burnham knows all about burnout. The American comic and writer was a certified prodigy at 16, one of the first YouTubers to go truly viral, with a million hits a day for his 2006 song My Whole Family Thinks I’m Gay.

By 18, Burnham had become the youngest comic to have a solo special on Comedy Central. Aged 19, he had an international tour and won the Comedy Awards Panel Prize at the Edinburgh Fringe. By 23 there had been a writing project with director Judd Apatow, a cameo in Funny People with Seth Rogen and Adam Sandler and three musical comedy albums.

Then, an enforced hiatus. “I had the first panic attack of my life on stage in front of 800 people in Edinburgh,” says Burnham, now 27. “I haven’t done stand-up in two years since. I always felt like an introvert who had a real passion to express myself, and that was the danger. Part of what I loved to do required me to do something very scary.”

All this is context for his writing and directorial debut, Eighth Grade, which is part of next week’s Sundance London Festival at Picturehouse Central. It’s a devastatingly funny coming-of-age film about 13-year-old Kayla, a middle school vlogger whose online advice posts to a non-existent YouTube audience belie her insecurities. Think Greta Gerwig’s Lady Bird, but brutally on the button, swiping through sexting, cyber-bullying, and that bottomless anxiety pit swallowing the Snapchat generation.

“I think it’s the most anxious time to be a kid,” Burnham says. In a way, though, the film is autobiographical — although Burnham says it’s truer to his adult self than anything he experienced at school. “I wanted to write about my own anxiety, I wanted to write about the internet, and it felt like my anxiety was tied to the internet,” he says. “The internet makes eighth-graders of everyone. It feels like the cultural conversation is existing on an eighth-grade reading level.”

His research was extensive. “I watched hundreds of videos of kids talking about themselves, videos of kids saying, ‘I’m going to teach you how to be popular’. And you listen to them for 10 minutes and you think: ‘Uh oh, you’re not popular’.

“It’s so clear they’re performing when they’re online, and they’re not being genuine. But in a way the performed version of yourself is the truer version of yourself. Who you want to be and what you want to sound like is probably a deeper truth about you than what you actually are.”

Are the kids all right, then? Burnham points to the platform given to the teenage survivors of the Parkland shooting as evidence things are looking up. “In the Sixties, culturally we realised we should take college students seriously. Now we’re starting to realise that we should take even younger people seriously too. Especially because they’re experiencing a world that we’re not experiencing. I really think the internet and phones rewire your brain in a different way. Their dopamine levels are different.”

This translates well into comedy, because the teenage experience is also farce. One scene sees a staged “shooting” being played out at a school drill (a real feature of the modern American middle-school experience), as children flick chewing gum at each other across a crowded hall and Kayla ogles her crush while being sprayed with fake blood by attendant teaching staff.

“Part of the movie is engaging with the broader culture — violent death and shooting — in the way it registers to a 13-year-old: as decorative stuff, as white noise in the background,” he says.

He leant heavily on his teen cast (rising stars such as Elsie Fisher, Luke Prael and Jake Ryan) to distil their experiences.

“Sometimes I feel movies about young people feel like some big statement from an older person, squeezing the kids into a vision that they have. I wanted the kids, the actors, to lead in terms of the texture and the truth of the movie.

“I don’t think this is a generation that’s self-obsessed or vain. It’s a generation that is self-conscious, in their own heads and nervous. It’s the responsibility of people in power to understand that.”

Eighth Grade is at Sundance London (picturehouses.com) on June 2 & 3