For some teenagers, wearing last season’s jeans will always be unthinkable. But a growing number consider texting on a dated smartphone even worse. For teenage apparel retailers, that screen-obsessed teenager poses a big threat in the still-important back-to-school sales season. Muscle shirts and strategically ripped jeans no longer provide an assured spot for retailers like Hollister and American Eagle Outfitters in the marketplace of what’s cool at an American high school. The social cachet these days involves waving the latest in hand-held technology. “Clothes aren’t as important to me,” said Olivia D’Amico, a 16-year-old from New York, as she shopped at Hollister with her sister and a friend. “Half the time I don’t really buy any brands. I just bought a pair of fake Doc Martens because I don’t really care.”