CHICAGO — Like the other fourth-graders at King Lab, Jennifer Dreller's daughter was discreetly weighed during gym class as part of a routine fitness assessment at the Evanston, Ill., school. But the experience took a toll on the 10-year-old's self-esteem, her mother recently told a panel of health experts.

CHICAGO — Like the other fourth-graders at King Lab, Jennifer Dreller's daughter was discreetly weighed during gym class as part of a routine fitness assessment at the Evanston, Ill., school. But the experience took a toll on the 10-year-old's self-esteem, her mother recently told a panel of health experts.

" 'How much do you weigh?' became the question of the month among fourth-grade girls," Dreller said during a forum on childhood obesity. "My daughter has cried many nights worrying about her weight since this experience."

As the nation's schools take an expanding role in the fight against obesity, they are increasingly flagging at-risk children with the help of an imperfect weight measure called the Body Mass Index, a ratio of weight to height.

But a backlash is building across the nation, sparked by concerns that BMI screening, which can be misleading, has no place inside a school. Critics also argue that putting the sensitive information in the hands of self-conscious young students can cause bullying, trigger eating disorders and intensify the pressure to diet.

Nevertheless, BMI remains the primary tool for classifying people as normal weight, overweight and obese. The Institute of Medicine specifically recommends school-based BMI screenings, in part because studies have found many students don't have access to a primary-care doctor.

"Schools have a changing role in wellness because that's where kids spend most of their waking time," said pediatrician Lynn Gettleman Chehab, who runs the school clinic obesity program at Evanston Township High School. "It's a crucial time for prevention, but many kids aren't going to the doctor unless they're sick."

The screenings are meant to cast a wide net, Gettleman Chehab added, acknowledging that BMI is flawed. "But if it's done in the right way, where privacy is protected and the child is not subject to embarrassment, it's useful information," she said.

Experts say obesity must be tackled at a young age, given the potential long-term consequences. Obese children have a 70 percent chance of being obese as an adult, putting them at risk for heart disease, type 2 diabetes, stroke, several types of cancer and osteoarthritis. Childhood obesity rates nationwide more than doubled to 18 percent over the past three decades, federal data show.

Simple, cheap and non-invasive, BMI is a widely accepted surveillance tool used to track trends in a population. The formula indirectly measures excess body fat - the real culprit behind a variety of illnesses and medical conditions - and a high BMI level correlates with future health risks.

Yet it can be problematic as a screening tool for individuals. Age, sex, ethnicity and muscle mass can influence the relationship between BMI and body fat. BMI also can't distinguish between excess fat, muscle or bone mass. As a result, athletes, muscular individuals and racial and ethnic minorities with different body compositions can have a high BMI.

About half of children whose BMI labels them as overweight (but not obese) are healthy and have no increased risk of diabetes or other conditions, said Kristine Madsen, an assistant professor at the University of California Berkeley School of Public Health and University of California-San Francisco Department of Pediatrics.

The Institute of Medicine recommended in 2005 that all schools annually assess their students' weight, height and BMI and send the information to parents as part of a national strategy to address weight problems in childhood.

Today, at least 19 states require school-based BMI screening. Nine states recommend it alone or as part of a broader health-related fitness assessment such as FitnessGram, developed by the Cooper Institute and licensed to at least 65,000 schools nationwide.

In Illinois, BMI data is required on school entrance health exam forms, along with vaccinations and dental and eye exams. But there's no statewide BMI surveillance system and obesity data are sparse, according to the public health department. At least 1,000 Illinois schools have incorporated the FitnessGram program, including Evanston.

Schools use and share the BMI data in different ways. Some notify parents of the results; others don't. In 2011, parents complained when Hawthorne Elementary School in Elmhurst planned to include BMI results as part of a physical fitness grade. The school hastily dropped the idea; today the information is reported to the state but not given to individual students. The Chicago Public Schools also reports only aggregated information.

Ideally, the BMI scores are used to help correct any misperceptions about weight and to motivate parents to follow up with a doctor. Multiple studies have found that parents of overweight or obese children often fail to perceive that their kids have a problem.

It's not clear, however, whether sending the information home does any good. In the largest study to look at the issue, Madsen and her team found no difference in pediatric obesity between kids in California whose parents who had received letters and those whose parents had not.

When the researchers dug deeper, they found the letters sent home were poorly designed, too complicated and lacked context, said Madsen. "I think that we just do not know what works," she said.

Parents also need information on how to respond appropriately to the BMI scores.

"We want to correct parents' perceptions, but what are parents going to do with that information?" said Dianne Neumark-Sztainer, a professor of epidemiology and community health at the University of Minnesota's School of Public Health. "My fear is that their intentions will be good but they may inadvertently do something that will be harmful."

Some groups that work to raise awareness about eating disorders oppose mandatory BMI reporting in the schools over concerns that it may trigger disordered eating in vulnerable kids. Experts, however, say that if one follows the other it is likely a coincidence of timing, as eating disorders typically develop during the pre-teen years or adolescence.

Moreover, obesity is the bigger problem, said Goutham Rao, clinical associate professor in the University of Chicago Pritzker School of Medicine and chair of the American Heart Association's Obesity Committee. "The proportion of children with eating disorders compared to those with overweight or obesity is very small," he said.

Rao said he frequently works with adolescent girls who want to be thinner.

"You show them their BMI and it's actually reassuring. They may be prone to a distorted body image and it shows, 'No, you're not overweight.' That's more common than pushing people in the other direction," he said.

As more schools instituted the screenings, their role has grown more controversial. In Massachusetts, the parental notification letters have been dubbed "fat letters." After the student-athlete son of a politician received a letter classifying him as "obese," legislation was introduced to prohibit the public health department from collecting data on students' height, weight and BMI. The bill has been referred to committee.

Dreller, who attended the meeting hoping to learn more about BMI, remains unconvinced that the measure should be calculated at school. Her daughter, one of the tallest girls in the class, weighed more than most of the others. That's something the 10-year-old hasn't forgotten.

"Aren't we trying to teach good body image and high self-esteem to our girls?" she asked. "Body image is so much a part of who a person is, that it seems like we're regressing."