In a glass-walled room inside Arlington's Yorktown High School, freshman Jon Comfort is playing with puppets.

Three mornings a week, Comfort and other high school students meet eight toddlers there. They read to the children, help them do puzzles, watch them in an observation room with a one-way mirror and a microphone. For the 3- and 4-year-olds, the program is nursery school. For the teen-agers, it's a chance to learn child development first-hand.

Yorktown's child development nursery laboratory has weathered nearly two decades of shifting emphasis in education, dropping from a high of 100 students enrolled 10 years ago to below 15 last year. But now, local educators say, the 17-year-old course may have a special, timely value in a society of broken homes, increasing awareness of child abuse and growing need for high-quality child care.

"First of all, there are a lot of life survival skills in child development," said Arlington School Board Chairman Gail H. Nuckols. "It is extremely important that we are able to train our young people to raise children."

As child abuse in families and in day care centers draws national attention, some home economics advisers say courses like the one in Arlington may help break the cycle by teaching teen-agers to understand children.

"We think it's going to help prevent child abuse," said Margaret Morris, program specialist for home economics in Fairfax County, where 15 of 23 high schools have programs like the one in Arlington.

And while an expanding range of glittery, high-tech careers beckons students, the child development lab may prepare them for the more basic job of parenting -- one that increasing numbers of adults are facing alone.

"I think one of the things that will go on forever, is that people will still be having children," said June Atwood, supervisor of home economics education in Arlington. "We are finding, too, that there are many people who are ending up as single parents."

Some high school students in the Yorktown class look forward to careers involving children. Some babysit frequently and want to learn more about the toddlers they care for. Others just enjoy spending time wih the children, who they say often startle them with unexpected honesty and humor.

"I was interested in psychology and sociology; also, I just wanted to get in contact with children," said Dalal Musa, 17, who took the class as a sophomore and has been an aide for the last two years. "I was sort of afraid of children because I never knew what they were going to say."

There was a lot of laughter in the airy, colorful laboratory one recent morning, as three toddlers manned a makeshift fire engine and a few others crouched behind a wooden stage for an impromptu puppet show.

Susan A. Fraser, who teaches the course, watched as the teen-agers handed each toddler a paper pumpkin and snips of black paper to make eyes and smiles. "Do we have smiling pumpkins or sad pumpkins?" Fraser asked the boy perched in a tiny chair to her left. "Benjamin, what do you have?"

Benjamin Strohl, 3, held up his black construction-paper crescent. "I have a moon," he said.

Because the atmosphere sometimes seems giddy, particularly to students who peer through the window as they pass in the corridor, the lab suffers the reputation of being a "goof-off class." But the students, teachers and advisers involved say the work is serious and valuable.

When people criticize the lab, "they're not examining what we're really doing," Musa said. "Play is the work of a child -- and they are learning from this." She said the value she and her classmates find in the course depends on how much effort they put into research and observation of the children. "If I went to a computer course and just sat there, that would be a soft subject," she said.

Musa is one of the course's most avid proponents. When low enrollment threatened to kill the class last year, she got 100 signatures on a petition to keep it in the curriculum.

Jon Comfort's parents thought it was valuable 10 years ago, when they enrolled their 4-year-old son in the nursery. The toddler was captured in the 1975 Yorktown yearbook, posing in an artist's smock in front of an abstract fingerpainting. Now Jon, 14, is the only boy taking the course. "Some people say things to me about being the only male in the lab , but it doesn't really bother me," Comfort said. "I babysit a lot -- it's helping me learn more about children and the way they act in different situations."

Jon's father, Joseph, said he thought his son's presence in the class was good for the nursery chidren, who rarely have contact with teen-age boys. "It might help some of them come out of their shells a little bit, because they identify with [Jon]," he said.