ABC News' Mary Pflum reports:

Lily Grasso, 11, is on the school volleyball team and eats healthy foods. So she was stunned when Florida health officials sent a letter suggesting she's fat.

"This whole thing is stupid," Lily, of Naples, Fla., told ABC News. "It can hurt people. It can break their courage."

"First I was hurt, and then I was angry, and then I just was concerned," said Lily's mother, Kristen Grasso.

The so-called "Fat Letter" is the result of a body mass index, or BMI, screening administered by officials at Lily's school.

"To give a kid a letter telling them the rest of their life they may be overweight or they may be obese because of a measurement you took one day, it's just not fair," explained Grasso, a mother of four.

'Fat Letters' Worry Students, Parents and Experts

In a bid to combat childhood obesity, similar screening programs have been embraced by schools in at least 19 states, from Arkansas to Illinois, who participate in the annual student weigh-ins.

"They're a great idea," said Dr. Stephen Pont, chair of the American Academy of Pediatrics' Provisional Section on Obesity. "I very much hope all parents can become aware."

But eating disorder experts worry the screenings do more harm than good.

"I would like to see BMI testing in schools banned," said Claire Mysko of the National Eating Disorders Association. "For those who are already insecure about their weight, these tests can … potentially trigger an eating disorder."

Young Girls Start Eating Disorders Early

Experts are concerned about the potential for eating disorders in girls and boys, but are especially in girls given that girls are more likely than boys to experience such disorders.

More than 40 percent of 9- and 10-year-old girls have already been on a diet, according to Duke University. And as many as 60 percent of all children between the ages of 6 and 12 years old are worried about their weight.

"Good Morning America" recently talked to other students who said they dread the screenings.

Zuzu Park-Stettner, 13, who attends public school in Manhattan, said of the BMI reports: "I hate them."

"It really doesn't do much for people except for make them more insecure about themselves," added Carmen Kunkel, 12.

3 Facts Families Should Know About Eating Disorders

Florida officials refused to speak on camera about Lily's report, but in a statement, they said the screenings, "Provide valuable information to parents and help ensure that Florida's students are healthy and ready to learn."

The statement also noted: "Notification of screening results is addressed to parents only and parents are encouraged to share results with their child's health care provider. Parents may choose for their child to opt out of the screenings by completing a simple form."

As for Lily, who weighs 124 pounds, she said she hasn't let the letter affect her and has learned an important lesson.

"Be confident in everything that I do and never give up," she said.