William Polly might look like any other 12-year-old kid among the throng of middle-schoolers who will swarm the halls of Jefferson Middle School in a couple of weeks. But the quietly poised young Arlingtonian is a Junior Olympics champion: an internationally renowned sport stacker.

Sport stacking, which involves the timed stacking of plastic cups in specific sequences, has grown increasingly popular since the late 1990s, with more than 480,000 competitors worldwide. While competitors can range in age from 3 to senior citizen, the sport peaks young, between10 and14. The sport, which hones hand-eye coordination and agility, is taught in many physical education classes in public schools across Arlington County.

William first started stacking in kindergarten, in an after-school program at Barrett Elementary. He got his first set of cups for Christmas in 2006 and started practicing in earnest in 2009. In April 2011, William won his first World Championship, becoming the youngest world champion and setting a world record. When sport stacking was added to the Junior Olympics, William became the first gold medalist. He has set 10 world records to date.

The world champion’s biggest challenge? Staying calm and stacking on. “At the World Championships, everyone takes out their camera to film you. Then, someone on the other side of the room sets a record, and there’s a lot of cheering. You have to set yourself off from everyone. You have to ignore your nerves,” William says.

William’s parents say the sport has brought William confidence and poise. “He can get up before a gymnasium full of kids to give a demonstration and not get rattled. I mean, the pressure of this sport is unreal,” says Kris Polly, William’s father.

The astounding wins have brought William guest stints on “The Tonight Show with Jay Leno,” “Live! With Kelly and Michael” and “The Steve Harvey Show,” and trips around the world. He is gracious and modest as he shows a visitor around his room, whose walls are lined with trophies and photos from his tournaments, as well as a set of stacking cups bearing William’s signature and his picture on a new tournament display timer box. He was featured along with other champion stackers in “Stacker,” a documentary about the sport. “Stacker” was shown at the Arlington Cinema ‘N’ Drafthouse in May to raise funds for Barrett and Jefferson schools.

The family traveled to South Africa this summer, where William filmed a television commercial for a South African orange soda. That was his favorite trip because his family went on a safari together. Next week, the young Olympian (and his mom) will fly to Beijing to attempt to break his world record in the “cycle” stack on a Guinness Book of World Records television show.

William shares his talent with other kids his age, visiting area elementary schools to teach and demonstrate the sport, advising those who want to start stacking clubs at school. He also likes to skateboard — to and from school — and play with a Rubik’s Cube.

During stacking season, which runs from late fall through midsummer, William practices three hours a day, doing homework after school before focusing on practice. In the summer, he cuts back to an hour or 90 minutes of practice each day.

The sport is not a solitary one, however. William’s friends cheer him on and even play along with him: “They think it’s really cool, and some of my friends are into stacking. So we’ll go to someone’s house, and just start stacking,” he says. His 16-year-old sister, Caroline, occasionally stacks as well, recently competing in the Junior Olympics; she says her little brother gives her tips.

Through YouTube and a Skype-like forum called Ooyoo, William connects with a far-flung but tightly knit community of stackers. “I’ve made friends with people all across the country and around the world. We’ll do video chats, and when we go to tournaments it’s so much fun to see them in real life.”

William’s mom, Laura, notes how supportive the community is. “They all Skype back and forth, so even though a lot of his close friends are from Ohio, North Carolina, New York, they are able to keep in frequent contact,” she says.

The Pollys would encourage other parents to support their children’s interest in the sport. “It’s the Everykid sport, is what it is. Every kid can do this. And a set of cups is only about $20. There’s no 5 o’clock in the morning on the ice for hockey, there’s no concussions, there’s no violence,” Kris Polly says. “It’s not a sport you have to buy expensive shoes for.”

Downsides? “The noise,” he says. “You know where they are in the house. No matter where they are.”

William advises kids who want to pursue the sport not to focus on winning. “Just try your best, and if you’re in it to win it, then you’re in it for the wrong reasons. If you’re having fun, that’s the best.”But if you’re not setting personal goals, and you’re not into it that much, then why do it? If you’re trying to impress someone, you’re totally in it for the wrong reasons — it has to come from you.”

For his part, the Junior Olympics champion and record-holder hasn’t let fame and awards go to his head: “I just want to keep stacking as long as I can,” he says.