It all began two years ago in a sixth-grade swimming class, when three boys saw "B.G." in her bathing suit and teased her about her weight.
Over the next year, according to the 11-year-old girl's mother, everything spiraled downward as she dropped to 96 pounds -- and eventually left her school.
"She had trouble walking, she couldn't carry herself anymore," said her mother, Mary V., who did not want their last name used. "Until then, her weight was proportional for her height. But after that, she chose not to eat."
The lawsuit, filed last week in the U.S. District Court for the Western District of Pennsylvania, alleges that the Pittsburgh Public Schools violated Title IX of the Education Act, which deals with gender equality. The lawsuit alleges the girl, who it refers to as B.G., was "harassed because of her sex."
The suit says the school created a "hostile educational environment" and names Frick Middle School and its principal, Wayne Walters, as defendants.
Pittsburgh Public School officials said they would "vigorously" fight the lawsuit and defended their anti-bully programs and harassment policies.
"We truly empathize with the family and certainly the student," said Lisa Fischetti, chief of staff and external affairs. "But in this case we certainly followed all of our standard procedures and protocols."
"We take bullying very seriously," she told ABCNews.com.
The lawsuit alleges that the harassment directed at B.G. was "sexual in nature," suggesting the girl was "unattractive and overweight" and that the school did nothing to prevent the bullying.
According to the CDC's National Youth Violence Prevention Resource Center, nearly 30 percent of youth in the United States -- more than 5.7 million -- are involved in bullying, either as a bully or a target, or both.
In a recent national survey of students in grades 6-10, 13 percent reported bullying others, 11 percent reported being the target of bullies and another 6 percent said that they bullied others and were bullied themselves.
Girls, in particular, are more likely than boys to report being targets of rumors and sexual comments.
Often, they are "anxious, insecure and cautious and suffer from low self-esteem, rarely defending themselves or retaliating when confronted by students who bully them," according to the CDC.
Such was the case with B.G., according to the lawsuit. She was taunted throughout the sixth grade at the magnet school, where she was a straight-A student, often making her cry, her mother said.
At first, her mother was unaware of the incidents and didn't notice the weight loss, she said.
"I thought every girl goes through ups and downs with body issues," Mary V. told ABCNews.com. "But the following year, after she went back to school, it was really dramatic."
When B.G. entered seventh grade in the fall, two more boys joined in the harassment, according to the lawsuit. While other students tried to shame the boys into stopping, the lawsuit adds, no faculty members or school administrators intervened.
"The boys would make comments about B.G.'s lunch, and would call her fat while she was trying to eat," the lawsuit claims. "This constant harassment eventually would affect B.G.'s health. B.G. began to throw her lunch away rather than eat in front of the boys and endure their constant teasing and comments about her weight."
In December 2007, during a field trip to a bowling alley, the same boys replaced her name on the scoring screen with the word "ugly," according to her mother.
When the teacher sent the girl to the school guidance counselor, B.G. was told, "Boys will be boys and they must like her if they were teasing her," her mother later claimed.
Shortly after the field trip, B.G. informed her mother of the teasing, according to the lawsuit. Her mother allegedly went to school officials and learned they knew of the harassment. A school investigation led to a one-day suspension for the boys.
By then, the weight loss was "significant," according to her mother, and B.G. began treatment at an outpatient clinic. Once the bullies learned it was an eating disorder, the teasing began anew, said her mother.
"She couldn't get the boys out of her head," said B.G.'s mother. "They controlled her thoughts with names and words. She drowned it all out with her schoolwork. It was her security blanket. She had 100s in every subject. It was the only way she could escape."
But by February 2008, B.G. already had lost more than 35 pounds and was admitted to the Western Psychiatric Institute and Clinic for treatment.
During the month she was hospitalized, the school was "very unsupportive," according to her mother, who said she arranged a tutor herself so B.G. could finish seventh grade at home.
Frick Middle School no longer exists and is now a grade 1-12 International Baccalaureate School. But in 2007, B.G.'s class participated in a program called, "Get Real About Violence," which had been in place for a decade.
"These behaviors start early on and in middle school the program teaches kids acceptable and unacceptable behavior and what to do when they are being bullied," said Janet Yuhasz, health services and student wellness counselor.
All of the principals, staff and teachers are trained in bullying issues, and the school offers an awareness campaign for its parents and students.
"We make everyone understand this is a very real issue and that there should be no fear in reporting it and there are fair and consistent protocols in place," said Fischetti, the schools spokeswoman.
But Edward Olds, an attorney for B.G.'s family, said even if the school had an existing bullying program, "it didn't work in this case."
"The bullying continued to go on," he told ABCNews.com. "Her mother finally got involved after the incident on the field trip at the bowling alley when it finally broke [B.G.]. When her mother tried to deal with it, they were not very responsive."
According to psychologists who specialize in anorexia but did not treat B.G., bullying many have been a "precipitating event," but would not be the cause of anorexia.
"The process was probably in place a long time beforehand," said Edward Abramson, professor emeritus in psychology at California State University and author of "Emotional Eating."
"Schools need to do more to curtail bullying," he told ABCNews.com. "But to attribute one girl's anorexia to a specific case of bullying -- lots of kids get bullied and don't become anorexic.
"Frequently, an anorexic girl was chubby in pre-pubescence and experiences some social pressure or a parent encourages them to lose weight or the pediatrician says go on a diet," he said. "The daughter initially gets a lot of praise, then it keeps going."
Michael R. Lowe, a professor of psychology at Drexel University, who is a research consultant to the Renfrew Center for eating disorders," agreed.
"Bullying is an awful cruel thing, but, unfortunately, common," he told ABCNews.com. "It appears that for a very small percentage of young women who have a vulnerability to anorexia, any weight loss brought on by any means could trigger this self-perpetuating pattern where losing weight makes them feel better not only physically, but emotionally."
In girls who suffer from anorexia, their biological behavior is the opposite of healthy girls who "immediately start trying to compensate and fight back," according to Lowe.
The disease is also thought to be genetic in origin and some studies show a connection between anorexia and anxiety disorders.
B.G., too, has struggled with obsessive-compulsive disorder and is still struggling with the effects of nearly two years of harassment, according to her mother.
"She's still very anxious," she said.
Today, she is of normal weight, is enrolled in a parochial school and continues to be a strong student.
"She's OK, but she has her good days and bad days," said her mother. "The bullying is a constant reminder that she can't escape. It's hard for her to trust people at her new school."
"It's been a long road," she said. "She's been through a lot, and I wish someone had told me about the bullying from the get-go. We had an open relationship before that, and it's hard to imagine how they had her mind so enthralled."