"He said, 'Oh, is that all?' "

Later in the day of the weigh-in, sitting opposite Ms. Thomas in her one-on-one therapy session, Carmen says it was her own decision to join her family at the Mexican resort as part of her "off-campus challenge." She does not regret it.

"Life can't be about a constant state of denial," she says. "You know my body gains weight so easily, and the weigh-in could have gone so much worse. ...

"It was my vacation and I didn't want to not drink pina coladas. You know - what about everything in moderation? I'm not going to make myself guilty, and think I can never have Haagen-Dazs ever again."

"You have to stop putting certain foods on a pedestal," Ms. Thomas answers her. "There's a whole big world out there ... and when you're not here, you have to be the control."

"Food is good. It tastes good. I want to enjoy my life," Carmen says. "It's terrifying to think I'll never be able to eat this again or eat that again."

Most experts agree that Wellspring's school program has several positive elements.

Jean Pierre Chanoine heads the pediatric division of the Canadian Obesity Network. He says it might be worth sending some kids to a boarding school "to get them out of a toxic environment." But he stresses that there is little concrete information about how long teens stick to the program after they leave the school.

After most interventions, 70 to 90 per cent of children tend to regain the weight they lose less than a year later.

Dr. Ball of Stollery Children's Hospital in Edmonton saw Wellspring's Dr. Kirschenbaum speak at a Vancouver conference on childhood obesity last year and was struck: "I've never seen another program able to demonstrate that 50 per cent can keep the weight off up to two years later."

But with no controlled study in a peer-reviewed journal, observers remain skeptical. "They should publish," Dr. Chanoine says. "It's a good initiative, but it needs to be evaluated."

The only article so far appeared last fall in the journal Obesity Management, reporting that more than half of students maintained their weight loss for 10 months.

Dr. Ball notes that weight loss with children presents particular challenges because parents have to be closely involved: They are generally the ones deciding what food is bought and prepared.

What's more, Dr. Ball says, some critics say it's inappropriate for children to count calories and focus on low-fat eating, as it may contribute to unhealthy obsessions and bad body image. "Some experts say it's better to focus on healthy eating and active living," he says.

Mr. Gordon says school officials present their data at peer-reviewed scientific conferences regularly. But conducting a large study with a control group takes "a lot of time and a lot of money."

Jesse Guth is one of the students Wellspring staff is happy to introduce, as the Toronto teen has maintained his weight since leaving the school in June, 2006. His 104-pound loss was so dramatic that he underwent surgery to remove the extra skin.

But he is the first to admit that his experience is extraordinary. Sprawled on the white sofa in his family's spacious North York home, he describes how his family has offered support and incentives. His parents encourage him steadily. They have a housekeeper, Emmy, who prepares nutritious, low-fat meals for the family. They have a gym set up in the basement. In the driveway sits a Mustang that is Jesse's so long as he keeps off his 100 pounds.

"We're in a position financially that we could afford to offer Jesse a reward," Ms. Guth explains. "He did once have to turn the keys in for a month."

"I can't say I enjoyed the program," Jesse says. "But based on my success, I would recommend it. ... More than anything, it gave me knowledge about what I was putting in my mouth."

The challenge, Jesse says, starts the second you get home and people stop weighing your food. He has stopped wearing his pedometer, though he continues to "mentally" self-monitor, he says.

Jesse has kept in touch with some former students by Facebook and estimates only three or four of the 25 kids he knew have maintained their losses. Most troubling, he says, is the fate of a boy named Jahcobie Cosum.