Gwendolyn Williams is a pencil-thin, bubbly 9-year-old who is a perfectly healthy third-grader.

But according to city bureaucrats, she’s practically obese.

“I was like, ‘Oh, my God! Why did I get this?’” the Staten Island kid recalled Thursday after getting a Department of Education-issued “Fitnessgram” that described her Body Mass Index as “overweight.”

“I’m 4-foot-1, and 66 pounds, and I’m like, what?!” Gwendolyn exclaimed of the school handout, which the city is sending home in the bookbags of 870,000 public school students, grades K through 12.

The kids, who were weighed and measured back in November, are told not to look.

But the Fitnessgrams are sealed with only a small, easily replaced round sticker — and peeking is rampant, parents complain, with devastating effects on kids’ self-esteem.

Gwendolyn’s mom, Laura Bruij Williams of Port Richmond, says she found out about her daughter’s Fitnessgram Wednesday night, as she was tucking the girl in for the night.

“She said, ‘Hey, Mom. The school told me I’m overweight.’ And then she started jiggling her thighs, and saying, ‘Is this what they mean?’”

“That was heartbreaking,” said the stay-at-home mom of two.

The next morning, Williams sought out Gwendolyn’s principal at PS 29.

“She was sympathetic, but said the kids weren’t supposed to open it. My response is, they’re kids. How can you believe they’re not going to open it?” Williams said.

“It’s a very positive thing for some kids who are overweight, but we shouldn’t be putting these assessments in the children’s hands,” the mom added.

“Fat-shaming,” experts called the practice on Thursday, criticizing both the fallibility of BMI calculations and the mental-health effects of kids being graded on their size.

“My friend who was next to me, she opened hers, too, and she was overweight too, and we were both saying, ‘Did the Fitnessgrams get mixed up?’” said Gwendolyn, who plays softball and loves to ride her scooter.

“I just don’t think that it’s fair to be called overweight when you’re not really overweight!”

BMI, while supported by the federal Centers for Disease Control and Prevention, was designed decades ago by the insurance industry as a way of assessing the health of groups of people, not individuals, said Chevese Turner of the Binge Eating Disorder Association.

“Dieting, especially for kids, is the gateway drug for eating disorders, and so is the public shaming that can come with this,” she said of the Fitnessgrams.

“My organization and others believe that BMI report cards have no place coming from schools and can be more harmful than helpful.”

A DOE spokeswoman defended the Fitnessgrams Thursday as “just one indicator … which helps students develop personal goals for lifelong health.”

But for Gwendolyn, the Fitnessgrams are just dumb.

“I know that I’m not overweight, so why should I believe the New York Department of Education?” she said.