Gavin Moynagh was having a not-so-great morning at the Timothy School. Battling a stubborn cold, the 16-year-old thrashed in a hallway and bellowed, "I don't feel very well!"

The thorniest part of Gavin's autism had won the upper hand.

And yet, none of the teachers and aides standing with Gavin so much as flinched as they worked to soothe the nearly 6-foot-tall teenager. This was a boy they knew well. A student at the Berwyn school going on 10 years now. The same one who, in more tranquil moments, harmonizes to music in perfect pitch or splashes and dives into deep water with his younger brothers.

"Anyplace else, I would have had to be explaining his behavior," said Kathy Moynagh, Gavin's mother, who had come to pick up her son for a doctor's appointment. But at the Timothy School, "they understand that these kids have good days and bad days, but the behaviors don't define their potential."

To Moynagh and others, the school is an autism refuge with remarkable longevity. It turned 50 this year, defying the odds through hard work and compassion as understanding of the brain disorder has grown. The school enjoys billing as Pennsylvania's oldest nonprofit exclusively serving autistic children.

It is not often that something that sprang into being through good intentions lasts for generations. But that's exactly the story that began when a Main Line church congregation formed the school in 1966, when little was understood about what would later be declared a neurological disorder.

The school has outlived its founding pastor at Bryn Mawr Presbyterian Church and its first home, years ago relocating from church grounds to a squat, old municipal building a few miles west in Berwyn.

And it's still growing.

"We are looking to launch a capital campaign in the next five years," said board chairwoman Deb Castle, whose son has been a Timothy School student for years.

It is an impressive trajectory for a school that began when there was no clear science behind the affliction, and there was a stigma in place of knowledge about how to best help autistic children.

Today, it serves students with moderate to severe autism, and there's a growing demand for slots. Its $55,000 annual tuition is paid by its students' local public school districts.

"They're really good," said Moynagh, who is grateful to have found such help for her boy, the oldest of three sons and the only one with autism. Gavin's challenges are on the more severe end of the spectrum.

Autism refers to a wide range of behaviors - some acutely debilitating, others less so - that result from brain abnormalities. It makes social interactions and communication difficult. People with autism also can be hypersensitive to sensory stimulation and tend to crave repetitive behaviors.

From autism's entry into the modern medical vernacular in the 1940s, scientific understanding of the disorder has deepened and government-funded supports have grown to help care for children and adults diagnosed with it.

The Timothy School, however, was a pioneer in what was uncharted terrain a half-century ago.

The school was born when Sunday school teachers at Bryn Mawr Presbyterian Church observed some children with such mystifying behaviors that they could not integrate with the other children.

Church staff reached out to the parents and learned that they, too, were overwhelmed. Mothers and fathers were caring for the children at home with no sense of what, exactly, was wrong or how to best deal with it.

The Rev. David B. Watermulder, a Princeton Theological Seminary graduate and then senior pastor at the church, pushed to create a structured classroom setting for them.

Watermulder had become pastor of Bryn Mawr Presbyterian in 1962 and brought with him a deep belief in its missionary spirit, said his widow, Ruth Watermulder. So he pushed for the Timothy School to get its start during weekdays in unused church classrooms.

"He was essential because he took it to our various board elders and deacons and was very affirmative in saying, 'This is something the church can do,' " Ruth Watermulder recalled.

The church paid a professional teacher to lead what would be a largely experimental school. It recruited worshipers as volunteers to learn from the teacher and assist with the kids. Parents also paid a share in those early days.

This was progressive, given the prevailing autism wisdom at the time.

In the 1960s, autism was wrongly presumed to be a form of schizophrenia. Another theory - also since discredited - placed blame on allegedly unloving, so-called refrigerator mothers.

Over the next few decades, the science behind autism deepened. The school evolved with it.

Moving to Berwyn 17 years ago cemented the Timothy School's commitment to grow, rather than fade away as a well-meaning mission of the church on Montgomery Avenue east in Bryn Mawr.

Meanwhile, the number of diagnosed cases was soaring nationwide.

In 2009, a state census found nearly 20,000 Pennsylvanians receiving autism services. By 2014, that number had nearly tripled to 55,000.

The Timothy School now serves 69 children a year, ages 5 to 21, from 26 school districts across Southeastern Pennsylvania and Berks County. Each classroom has four to seven pupils.

Staff turnover is low, a fact that school officials and even some staff members credit to a familial vibe that's been present since its founding, even as it has implemented scientifically sound educational strategies.

This was apparent in a visit last week, where the board chairwoman stumbled upon her son, Drew Castle, heading into a training kitchen where freshly baked trays of cookies awaited him.

"Come here. I need help," teaching aide Marian McCaughan told the teen in a firm but tender tone. "Did you wash your hands yet? Come on over."

McCaughan coaxed Drew, a smiling, wiry 19-year-old, to the sink. "Roll your sleeves up. Let's wash your hands."

McCaughan, one the 70 paid staffers, has been a teaching assistant for eight years.

"The woman I work with has been here 32 years!" McCaughan said. "When you're here that long, you love the kids. You're here because you want to make a difference. That's why I'm here."

Deb Castle was recruited to join the board six years ago as a parent with strong advocacy skills. At the time, it was still predominantly composed of church-affiliated members.

But Castle, a former corporate human resources executive, sensed that new members would bring educational credentials and corporate-world contacts that could attract much-needed sponsorships and philanthropy.

"We needed a change," Castle said.

The school bought three nearby houses in recent years and turned them into school buildings. And earlier this year, the board hired a new executive director, Matt Riley, to replace Judith D'Angelo, who retired after two dynamic decades. Riley brings a business and special-education background, key for the next phase of growth, which could involve building a new school.

"The foundation of the school," Riley said, "is very solid."

David Watermulder died last year. He would have been amazed to attend the school's anniversary gala in November, said his widow.

"I was really overcome with emotion to realize where this had all gone, from its humble beginnings, and only wishing my husband had been here to see it," Ruth Watermulder said. "Because it was his vision."

mpanaritis@phillynews.com610-313-8117@Panaritism