Holding cameras and copies of her new album, When the Sun Goes Down, they intend to wait until the Texas-born actor-singer leaves for an instore appearance. Then they'll race to the store themselves — they've already got the blue wristbands required for entry — and get Gomez to sign the album.
"She's in my top favourite group [of pop stars]," says Jemma, eyeing a couple of loitering paparazzi disapprovingly. "I'm seeing her [play] in LA on the 24th too — I'm making my parents stay in LA longer than they wanted to so I can see her."
Sophia, who says she's 14 but looks younger, tries to define what it is about Gomez that made her bunk off school (with her mother's permission, she maintains): "I like that her songs are really inspiring, she does a lot of charity work and she seems like a normal person." The other three nod.
There's no starry mystique about Gomez — she seems normal, approachable, the kind of girl they'd like to be friends with. That counts for so much with people in her target age group (nine to 14) that she's currently the biggest star attached to the Walt Disney Company — the US entertainment conglomerate that identified the tween market a decade ago and made household names of Hilary Duff, Miley Cyrus and the Jonas Brothers.
When the Sun Goes Down is her third LP of amiable powerpop in less than two years, which says something about how assiduously she's exploiting her time in the spotlight. The same period has also produced a Disney TV series — Wizards of Waverly Place — and no fewer than six movies, ensuring that fans have plenty of Gomez in their lives. Naturally, she's also busy tweeting and Facebooking, and has lent her name to the inevitable perfume and fashion ranges.
All this means that tweens can barely avoid her, and that goes double now that she's dating the world's most screeched-at pop star, Justin Bieber. The two of them have been photographed splashing in the Pacific surf and posing like a pair of fluffy puppies on red carpets, which has led to Gomez being deluged with hate-tweets from heartbroken Bieber fans, aka Beliebers.
Thea, 13, is a Belieber but also loves Gomez, a state of affairs she doesn't consider paradoxical: "I don't mind her having Justin, 'cos I'll have him one day," she says, with commendable confidence.
And how does Gomez herself feel about being on the receiving end of so much Belieber bile? She shamelessly evades the question. "I just focus on everything that's going on with my fans."
But, she adds, she knows what it's like to have a teen crush: "I was going to marry [American pop singer] Jesse McCartney — he was my screensaver. And I've been obsessed with Shia LaBeouf since I was 15." (A whole four years — she turned 19 last week.)
She's been installed in a swish, dimly lit meeting room. Two publicists sit in on the interview, though heaven knows why — she's been so diligently media-trained that she's unlikely to drop a clanger. She's almost preternaturally poised: hands folded in her lap under the artfully draped folds of her black dress, she resembles nothing so much as a china figurine.
As she talks about life as queen of the tweens, her expression is unreadable. She bats away any question that diverges from her script, so whatever her thoughts on, say, teen stars going off the rails — witness the recent antics of fellow Disney acts Cyrus and Demi Lovato, which landed Lovato in rehab — she's not sharing them.
"I have a really great support system around me, and that keeps me straight. It gets overwhelming at times, the scheduling, with touring, acting, my clothing line, music — there are times when you almost break down. Luckily, my mum is my manager, so I've always got her with me."
She says she's happiest having younger fans. "I get on better with kids than kids my own age. I'm growing up, but they're growing up with me. We all go through the same things, feeling awkward in your body and stuff."
Does she think dating Bieber is distracting attention from her work? "I work really hard at the things I love and that's all I really want to be known for."
You are, I suggest, not saying what you're really thinking, are you? Her serene smile widens and there's a slow, triumphant nod. She has the presence of someone twice her age. It's hard not to feel that Bieber is way out of his league with her.
‘Level-headed and gracious'
And it's easy to see why Disney executives love her. On the phone the following week, Disney Channels senior vice-president Adam Sanderson practically falls over himself praising her: "Selena sees herself as a role model. She's not only a fantastic performer and wonderful person, she's level-headed and gracious. Her crowd absolutely adore her, and mums love her, too."
Tween-and-mum appeal — that's what Disney and rival family-entertainment companies such as Nickelodeon (which has its own TV-show-spawned-popstars in Big Time Rush) look for, because when they get it right they're rewarded with the passionate devotion — however long it may last — of girls like Jemma and her friends. (One of her worst moments, Jemma says, was when, as punishment for bad behaviour, her dad tore up her tickets for a Gomez gig in London last year. That must have hurt, I say, and she nods with great feeling.)
Jemma and the others are part of a demographic that has been making a great deal of money for Disney, which shrewdly realised there was an audience out there that was "too old for Nickelodeon and too young for MTV", as Disney Television president Anne Sweeney once put it.
The company has targeted them with a slew of music-themed films and TV series, including High School Musical (which inspired the Fox Network's stormingly successful Glee, incidentally), Hannah Montana (starring Cyrus, and which Gomez appeared in) and Gomez's series, Wizards of Waverly Place, which has just ended after four seasons.
All of them starred then-unknown young actors — unfailingly referred to as "the talent" at Disney HQ — who were tirelessly cross-promoted through the company's TV, film, radio and record divisions. (The Jonas Brothers, who started their career independently, also eventually found their way to Disney.)
Gomez's career trajectory is typical. Minor TV work from the age of seven led to her being signed by Disney and cast as the lead character — a teenage magician called Alex — in Wizards, which debuted in 2007.
It became an Emmy-winning hit, and Gomez herself won several Teen Choice and Nickelodeon Kids' Choice gongs. At the same time, she began making albums for the company's Hollywood Records imprint, and she didn't have to worry about getting airplay, because, handily, there's a Radio Disney — and a ready-made audience among those who watched Wizards.
Disney's acts also have the option of performing live at Disneyland in Los Angeles or Florida's Disneyworld (a Christmas show at the latter by Cyrus is credited with helping to establish her).
The idea of one monolithic company influencing every aspect of an artist's professional life is, depending where you stand, either Big Brotherish or just the way things are in an era of multi-platform 360-degree deals. Disney espouses the latter view, obviously. What it does, says Sanderson, is "stitch together all the pieces strategically to build affinity around the content".
Well, that sounds impressive. But can he remember the days before marketing execs talked this way? Who were his own teen idols? He laughs ruefully and admits he was a fan of The Partridge Family, the '70s sitcom that made a star of David Cassidy. "I had the lunchbox! [The 360-degree deal is] a modern version of that."
The Partridge Family were an early example of — if you must — building affinity around content: in addition to the TV show, there were albums, tours and lunchboxes, turning Cassidy into the Bieber of his day.
Along with The Monkees' TV series, the show paved the way for the music-themed programmes of today. The difference between then and now, of course, is the global scale of today's assault on the tween piggybank. Disney broadcasts in more than 150 countries, potentially giving access to billions of kids and, through them, their parents' wallets.
"It's not about catching them young. It's about what kids want, not just what Disney thinks they want. They want to connect with [the stars], they engage with the characters. They don't think about one area of the business versus the other, it's all about watching TV and going online and listening to music. It's part of their world," Sanderson says.
And what happens when the talent grows up and no longer relate to their audience, or want to sing the zippy powerpop tunes Hollywood Records sources for them? "There comes a point when they decide they want to do other things, and we don't stand in their way."
Even wholesome Gomez may be reaching that point. "There's only so long you can play a high-schooler," she says of the cancellation of Wizards of Waverly Place. She's happy with her good-girl image, she hastens to add; she just wants "to do things that are a bit different for me".
Let's hope Jemma, Sophia, Thea and Katy aren't too disappointed.