Shortly after school began in the fall of 2004, an eighth grader named Tye Clark delivered a jolting message to her classmates.

"I'm the same person I was last year and the year before, but I am transgender and will now come to school as a boy," Tye told four assemblies at Cedarbrook Middle School in Cheltenham Township. "You may not agree, but I hope you will respect me and my right to get a good education."

Eyes misting with tears, Tye asked to be known as Ty. The transgender teen finished to rounds of applause.

Ty celebrated his 18th birthday yesterday. He was born female, but as far back as he can remember, he felt in his brain and his heart that he was a boy.

For years, his father, a family physician, and his mother, a life coach, resisted their child's yearning to switch genders, hoping Tye would grow out of it.

But the feelings intensified when she reached her teens.

"I hit the wall," he said at his family's house in Wyncote during one in a series of interviews. "I began to develop breasts, and when my periods came, it was the most horrible time of my life."

Now, nine months into twice-monthly injections of testosterone, administered by his father, his voice has deepened and there is fuzz on his face.

The high school senior says he's much happier. He has male friends and, with his dark good looks and friendly charm, has no trouble meeting girls. He says he's always upfront about his past.

No one knows how many people feel trapped in a body that they insist has betrayed them, but experts say the small number of children and teenagers who are coming out as transgender appears to be on the rise.

Psychologist Kenneth J. Zucker, who heads the gender-identity service at the Center for Addiction and Mental Health in Toronto, says the number of gender-changing adolescents in his clinic has tripled over the last four years.

Mental-health experts say that word about the condition is spreading - largely through the media and the growth of advocacy groups - so more transgender children feel comfortable talking about it.

The medical community, too, is evolving. TransYouth Family Allies, an online support and education group, has compiled a list of health care professionals who will work with transgender children and their parents.

A scattering of doctors such as Norman P. Spack, a pediatric endocrinologist at Children's Hospital Boston, now follow a model from the Netherlands, where children who believe that they are transgender may be given reversible puberty-blocking drugs.

These allow them to take a few more years to consider their options - and be spared the traumatic experience of puberty in a gender they reject.

Some doctors are on the fence.

"I would try to work with a person who is gender dysphoric to help him or her adjust to his biological sex, but by the time a child is 13 or so and has been consistent and unflinching about who he is, it may not help," says Main Line psychiatrist Fred Fisher. "Still, I'm not going to be a gender-switcher."

Even proponents generally decline to switch children under 10, believing that is too young to make such a momentous decision.

The American Psychiatric Association's bible, the Diagnostic and Statistical Manual of Mental Disorders, was last revised 18 years ago and refers to transgender cases as a "gender identity disorder."

But revisions are now under way, and mental-health experts are debating whether the condition should be included in a manual of psychiatric disorders at all.

"These kids are not mentally ill," Boston's Spack says. "It has something to do with the wiring in the brain - maybe a gene that is expressed at a certain stage of fetal development or hormones gone awry during gestation."

Transgender children and their families have an ally in the American Medical Association, which in June urged insurance companies to cover the costs of treatments, whether puberty-suppression medication, psychotherapy, or sex-reassignment surgery.

Transgender has nothing to do with being gay or lesbian. Nor is it like being a cross-dresser, a man who gets aroused by wearing silk stockings or lace panties.

People who are transgender feel that nature made a mistake and gave them the wrong body parts. They merely want to live in the opposite gender and not be viewed as freaks.

Distress is too tame a word to describe the anguish that families of transgender children say they feel.

"To know your child as one gender and have him tell you he's another is overwhelming," says Stephanie Guinan, the trans coordinator of PFLAG Philadelphia, a support group for parents and friends of lesbians and gays.

"Through the years, they become depressed, fail in school, and try to self-mutilate." A study published in a 2006 Journal of Homosexuality found that a third of 515 children surveyed had contemplated suicide.

"My child was suicidal," says the mother of a 12-year-old who was born male but identified as female. "As soon as we let her wear a dress in public, she was a different person." She asked not to be identified to protect her child from ridicule.

"When you get a kid who is morbidly depressed or wants to kill himself, to me, it's a no-brainer," says Michele Angello, a psychologist in Wayne and Doylestown, who specializes in gender issues.

"You help the family understand that transitioning will turn everyone's life upside down, but trying on the desired gender identity, without doing anything irreversible, is worth considering when a child's psychological health is at risk."

At the Clark home in Montgomery County, Ty's parents say they endured 12 years of anger, frustration and tears while bouncing from therapist to therapist. They even enrolled her in an all-girls school so she could see that girls can express themselves in various ways.

They believed that their child would probably grow up to become a butch lesbian. Tye, meanwhile, kept insisting her body didn't fit who she was.

"I thought God was playing a joke on me, and after puberty I would turn out to be a boy," he recalls.

"For a brief time, Tye conceded, 'OK, I'm a girl,' and she tried to adapt," says Matthew Clark, who was determined that his oldest child would grow up female.

"But the fall she turned 13, she began to freak out. What she felt on the inside and what was happening to her on the outside was tearing her apart."

His mother, Rory Cohen, remembers "the irony" of it all: "I was buying her boys' underwear and Kotex on the same day."

That's when they decided to let their daughter make a "social transition" into their son.

The family agreed that Tye would go through her bat mitzvah as a girl - and then they would talk with the counselor and principal at school about removing a letter from her name and switching the gender on school records. She would come to school in boys' clothing and use the nurse's bathroom.

Officials were sensitive and cooperative. Tye was most worried about how students would react. But after the assembly speeches, they congratulated him.

Classmates and their parents aren't always that accepting. In April, the Haverford Township School District administration sent letters to parents explaining that students would be told at a meeting the next day why an 8-year-old classmate, a male, would now come to school in girls' clothing and be called by a girl's name.

The protest from parents was vigorous. Some asked that their children not attend the meeting. Others wrote angry notes on the township blog. Asked one parent: "Why is the school introducing this subject to eight and nine-year-olds?"

"It isn't easy," says Ty's father. "I still feel uncomfortable when I tell people I have a transgender child and this is how I choose to raise him. I wonder if they go home and say, 'Those people must be out of their minds.' "

Ty had a double mastectomy last year; before then he had been wearing layers of T-shirts to camouflage his breasts.

"I was nervous about the surgery," he recalls. "But excited beyond belief. I counted down every day on my calendar. All I could think about was getting the best male body I could."

Since then, Ty says his life has flipped from despair to joy. He feels comfortable in his clothes. He doesn't have to hide his body anymore. And he has never been threatened or bullied, as are many transgender children and adults.

He sees marriage and children in his future, perhaps through use of a sperm bank or adoption. He hasn't yet considered additional surgery.

"For the first time in my life," he says, "I like the body I have."

Links to resources and information about transgender issues: http://go.philly.com/healthEndText

Contact writer Gloria Hochman at gloriahoch@comcast.net.