GREENSBORO, Vt. – Fourteen-year-old Thula Martin knows exactly what she’ll say if her teacher asks the class what they did this summer: “I ran away and joined the circus.”

She did, sort of: Thula, a ninth grader from Pahoa, Hawaii, is one of 30 teenagers from around the world spending their summer performing in Circus Smirkus, a youth circus that barnstorms the Northeast each summer, showcasing school-aged acrobats, jugglers and clowns under a one-ring big top.

It’s not the greatest show on earth, but it may be the youngest: In this circus, everyone in the ring is between 14 and 19 years old. It’s like summer school with greasepaint.

Each June, a new class of talented young entertainers shows up at the converted dairy farm with the blue-and-white circus tents, where veteran circus coaches get three weeks to build a show around them before the troupe hits the road for a 71-date two-month tour of New England and New York.

Founded by a former circus clown, it’s named after the comment made by his mother, after he ran off to join a circus at age 19 (“Circus, smirkus, get a job.”).

Rob Mermin decided other teenagers should have the same experience, and pitched his tents on an eight-acre hillside deep in northern Vermont, near the U.S.-Canadian border.

Twenty-one years later, Circus Smirkus still has `em laughing in the aisles, and the circus business is full of its graduates.

“It’s intense, but it’s nice because you have all these other people doing the same thing as you, so it kind of pushes you to work harder and get better,” said Shea Vaccaro, 16, an acrobat and clown from Wilton, N.H. “And the coaches are great. They push you, but it’s to the good point.”

Up to 400 children a year apply for spots in Circus Smirkus, and the ones accepted pay $4,250 for the privilege. Most have been performing since they were toddlers.

All get the same treatment once they take rugged Circus Road through the woods to the grassy encampment at the end, where the center of operations is a ramshackle 180-year-old barn and farmhouse.

Living in beat-up trailers, a baton’s throw from the tents where they practice, the young entertainers help cook the food, wash their own plates and use a makeshift outhouse (marked with a hand-lettered “Super Trouper Pooper” sign) when nature calls.

They train under the tutelage of veteran circus performers and coaches, and abide by some strict rules. Use of alcohol or drugs is grounds for immediate dismissal, as is sex. But most need little outside motivation to keep the clowning around where it belongs, according to Ed LeClair, executive director.

“If you say `You’re not in the ring next time,’ they’ll (ask) `How clean do you want these pots?”‘ he said.

On a recent Tuesday morning, all 30 started their day with a warm-up routine in the big tent, among them Book Kennison, 19, of St. Charles, Mo.

Kennison, an impossibly elastic 6-foot-3-inch 150-pound juggler and contortionist, combines the two into some eye-popping juggling tricks when he’s not wowing audiences by squeezing his body through a tennis racket – no strings attached.

This year’s two-hour show, “Smirkusology: A Science Extravaganza,” kicked off June 28 with a pair of shows at the Vermont home base, with many of the performers’ parents on hand – working as face painters, ticket takers and ushers until the curtain went up.

Not everything was ready for prime time: a carabiner in her rigging got tangled during the middle of her aerial act, grounding her temporarily. There were a few dropped juggling pins, and some timing glitches.

But the small circus rollicked, its two-man band high above the 750-seat tent, playing original music while the crowd clapped along. Toddlers and small children, rapt at ringside, sat on their knees in the sawdust, just outside the ring.

If history’s any indication, some will be inside the ring some day. Many Circus Smirkus alums started as wide-eyed fans.

Trouper Isabelle Schuster, 18, of Viery, France, whose mother runs a circus school, started as a toddler after watching her older sisters ride unicycles.

“I got really jealous, because they could ride a unicycle and I couldn’t,” she said, so she learned to juggle and ride.

Now? “I do circus all the time,” she said. “When I’m mad, I’m going to do circus. When I’m happy, I’m going to do circus. It’s just something I like doing.”