October 22, 2014 12:00 PM

THE LINE WAS HALF A MILE LONG. “LUKE, HOW Tight Are Your Pants?” and “I Like…Boys With Accents” signs popped up from the crowd of thousands. Some fans had even camped out for six days to see 5 Seconds of Summer storm the Today show stage. In fact, hysteria ran so high that 18 people actually needed medical help afterward. “This is probably the biggest crowd we’ve had,” said Al Roker. “This is One Direction-size.” So was the roar for the four pop-punk heroes from Australia: the Charmer, Luke Hemmings, 18; the Rebel, Michael Clifford, 18; the Introvert, Calum Hood, 18; and the Sensitive One, Ashton Irwin, 20. Girls squealed, sobbed and screamed “Marry me!” at the lanky lads—they’re all at least 6 ft. tall!—as they rocked out to hits like “She Looks So Perfect” and “Don’t Stop.” Said Ashton: “Seriously, this is the most incredible thing that’s probably ever happened to us. It’s amazing.”

Just as amazing? Their very first album, 5 Seconds of Summer, debuted at No. 1 on the Billboard 200 in July, beating the Frozen soundtrack with 259,000 copies sold in one week. 5SOS’s videos have gotten more than 150 million hits on Vevo, and the band has more than 4 million Twitter followers and counting. After another whirlwind tour with One Direction, they’re pros at playing to sold-out arenas like London’s 90,000-seat Wembley Stadium. 5SOS even gave American Apparel a boost: Thanks to the catchy chorus of “She Looks So Perfect,” which reached No. 1 in 39 countries on iTunes within two days of being made available for preorder, sales of that brand’s underwear have spiked 10 percent.

For the world’s hottest band, it feels like only five seconds ago that 5SOS (pronounced “five sauce”) was just something they dreamed up in school one day. “It still takes time where it hits, like ‘Is this real?’ ” Michael told 60 Minutes Australia. “As a band, sometimes we might look a little bit cool,” Luke has said. “But we’re just dorky dudes.” In music class at Norwest Christian College in Riverstone, a small town near Sydney, he, Michael and Calum bonded over a love of Usher and pop-punk bands like Green Day and Blink-182. They decided to start a band and began posting YouTube videos of themselves singing Jason Derulo and We the Kings tunes. When they needed a drummer for a show, Michael Face-booked his pal Ashton, feeding him a little white lie that they’d be playing for 200 people. “It was 12, actually,” Ashton told Billboard of that December 2011 audience. “But I still say it was my favorite gig ever. We had amazing emo fringes.”

Soon after, 5SOS caught fire when their cover of Chris Brown and Justin Bieber’s “Next to You” got more than 600,000 views on YouTube. The band was even more pumped when “Try Hard”—a song written by Calum and Luke—landed them an opening gig for Hot Chelle Rae’s Australian tour in October 2012. The next month, 5SOS won the YouTube lottery: One Direction’s Louis Tomlinson tweeted a link to their cover of “Teenage Dirtbag” by Wheatus. “Been a fan of this band for a while,” Louis said. “Everyone get behind them.” Suddenly, 5SOS had tens of thousands more followers, and in February 2013, 1D invited 5SOS to join them on their Take Me Home Tour. “It was a really big opportunity,” Luke has said. “We weren’t that good at the time, and we got to become a better live band and put our music out.” The guys became fast friends—and pranksters. Who can forget the onstage pie fight in Australia last October? And backstage “there’s usually nicking of golf carts,” Tomlinson tells PEOPLE. Adds Liam Payne: “We like to turn everything upside down in their dressing rooms.”

With Harry Styles and the 1D guys in their corner (and their green room), it’s easy to confuse the two groups. Heck, Kendall Jenner nearly introduced them as One Direction at the Billboard Music Awards in May—of course, by August 5SOS was so huge, she was spotted out with Ashton! But please, pretty please, don’t call 5SOS a boy band. Sure, they’re totally adorable and their hair is poster-perfect, but (1) they play their own instruments; (2) they cowrite their songs; (3) they weren’t put together by Simon Cowell; and (4) the “boy band” label gives them sad face. “It’s a little offensive,” Michael has admitted. “People get confused because we’re young and we have a female fan base,” Ashton has said, adding that pop-punk band Fall Out Boy did too. “Pete Wentz was the Justin Bieber of 2007. Girls obsessed over him.”

Next the fab foursome will play three dates in the States in November, followed by headlining their own Rock Out With Your Socks Out Tour throughout North America, kicking off in Las Vegas next July. Life on the road means epic Nerf-gun fights, movie marathons (Guardians of the Galaxy and War of the Worlds were recent picks) and regular texts to Mom—or “Mum,” as they say. “Really do miss home, my family, friends,” says Ashton. In the meantime the guys are writing new music for a second album. “I’d like to do some sorta acoustic album recordings one day,” says Ashton. And Will Smith, if you’re listening, 5SOS would love to record something with you. “The sky’s the limit,” Michael has said. Whatever happens, they’re wicked happy about right now, whether they’re winning their first MTV Moonman, autographing breasts or firing up stadiums packed with ob-SOS-sed fans. “When three years ago I was playing to a garage door,” said Ashton, “playing to 50,000-plus people a night is just the best feeling ever.”

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