Hammering. Sawing. Sweating. A group of teen-age mothers is learning the hard work of parenting and independence.

Handling the hammer like a veteran carpenter, 14-year-old Karen Clark offers a no-nonsense lesson in how to pound nails.

“It’s all in the wrist,” she says matter-of-factly at a Pacoima construction site. “Look, you move your whole arm and your arm gets tired. You might miss the nail and hit your thumb. Use your wrist and you can hammer all day long.”

During the past three weeks, Clark has learned a few things about building houses: how to erect scaffolding, caulk a joint and earthquake-proof a home.

She’s also gleaned a few lessons about being a better mom: how to be a bit more patient and how to discipline her year-old daughter, Blanca Rosa, without striking her. Most of all, however, she’s learned that being both a teen-ager and a mother is not the end of the world.

Clark isn’t the only mama on this work site. All around her--nails in their mouths, hard hats on their heads--are other teen-agers in the same precarious boat, young women who have barely reached puberty but who suddenly have this demanding new weight to shoulder: a child who expects them to be Motherly. To be an Adult.

This month, 84 students from an Detroit public school for pregnant and parenting teen-agers--more than half with infants and five still breast-feeding--have come to Los Angeles to help rebuild homes in low-income areas, while doing a little construction on their own self-esteem.

The project--known as Away School--is the brainchild of Asenath Andrews, principal of the Catherine Ferguson Academy, who wants to teach students the living skills needed to make it on their own. During the past two years, they have camped in Yellowstone National Park and helped rebuild a day-care center in Washington, D.C. This year’s trip was funded with both public school monies and a $70,000 educational grant from the state of Michigan.

“These girls are learning that the world is bigger than the local mall or their own neighborhoods,” says Andrews, 42. “They’re stronger than they ever thought they could be. They’ve put up roofs, seen whole houses take shape before their eyes. There’s nothing more powerful than that.”

They’ve also learned a valuable lesson in sisterhood.

“When a girl is on top of a two-story building and she needs nails, and looks down to see that the one handing her the nails and the one holding her ladder are both women, she tells herself ‘Hey! I can do this!’ ” says the soft-spoken Andrews.

“They don’t have to call on any boyfriend or husband.”

*

In early May, Andrews and her charges left Detroit: a caravan of three chartered buses, two chase vehicles, 81 young mothers, three fathers, 45 infants, 11 nursery workers, eight teachers, a counselor, nurse and two administrators.

For eight days, they meandered through small and large cities, past natural and historical sites, under yawning Western skies and over roaring rivers--images that glued the young women to their bus windows.

They passed Chicago, the Wisconsin dells, Custer Battlefield National Monument in Montana, the Grand Canyon, Hoover Dam, Las Vegas and Mojave Desert. All the while, teachers conducted history and biology classes that corresponded with the world directly outside.

The students, ages 13-19, kept journals of their experiences. And they spent an hour each day just holding and reading to their babies--without handing them off to a mother, sister or boyfriend.

Mostly, though, they just watched and wondered.

Several wept at the majesty and immensity of Mt. Rushmore. In her journal, Shalonnia Frazier, 18, described how “the afternoon sun hit the snow-capped peaks of the Grand Tetons, making them look silver.”

And one girl, considered to be the toughest and most streetwise among her peers, squealed in fear as she approached the rim of the Grand Canyon.

They saw sights unknown to the inner city: Driving for miles in Wyoming without passing another person or car. Towns with populations of five or 10. Signs that warned of no services for the next 100 miles.

Andrews likens the trip to a version of “National Lampoon’s Summer Vacation”--with babies. Moments were rare when the children--ages 2 months to 3 years--were quiet in unison.

Teachers squabbled with bus drivers who wanted to show such on-board movies as “Animal House,” rather than more relevant and educational films such as “Little Big Man” and “Dances with Wolves.”

The group stayed at old motels where the water ran rusty through aging pipes, causing the girls to cry out, imagining the water was tainted with blood and they were part of some Stephen King thriller.

One day, when the buses were stranded without the food-carrying chase vehicles, teachers were forced to make a concoction of soup and pasta using clean, never-used garbage cans for pots and huge branches for spoons.

The girls called it Tree Branch Soup.

*

Once in Los Angeles, the group checked into the American Youth Hostel in Santa Monica and got to work at Habitat for Humanity projects in Long Beach and Pacoima.

Their schedules bear the hectic quality of some teen-age boot camp. Each weekday, reveille comes at 5:30 a.m. Then it’s off to the the beach for a half-hour of exercise. After breakfast, vans shuttle the students to the work sites.

The three fathers work right alongside the mothers--some issuing orders to the young women, others taking them, all learning about proper respect for the opposite sex.

Back at the hostel, the nursery workers tend the babies. At night, the girls take turns cooking huge family-style dinners. Some then attend art, history, biology and computer classes in the shady hostel courtyard. Bedtime is 10 p.m. sharp, every night.

So far from home, Andrews has been able to better control the haphazard lives that cause students to miss school or ignore assignments. On this trip, she calls the shots on what the girls eat and with whom they associate.

Discipline problems have been few. Two girls who left the hostel without adult escort were forced to stand facing the wall for an entire day. And missing morning exercise brings such penalties as cleaning garbage cans or scrubbing floor tiles.

Meanwhile, the girls have fought bouts of homesickness, fast-food withdrawal and the mood changes that plague most teen-agers. And, of course, the challenges of sleeping in bunkhouse-style rooms, some of which hold 27 mothers and their babies.

Boiling bottles for newborns and laughing as they wage water-balloon battles, the young women walk the tightrope of being attentive mothers and children themselves.

“I think these girls have learned to become more appreciative of their own mothers,” Andrews says. “When you have to take care of someone so young, the nurturing you need as a child often doesn’t come.

“But many of these girls are still children. They need attention. Out at the work site, I’ve had them come whining for a Band-Aid or ice pack. They’ll come up and say ‘Can you take this splinter out?’ Meanwhile, they’re walking around feeling sorry for themselves, like they have a tree branch stuck in their finger or something.”

The girls respect Andrews as both a friend and mother figure. Each night, she takes a count of girls who feel ill and whips up a batch of her “Witch’s Brew” tea. The patient principal wanders hostel hallways at night, breaking up arguments and calming screaming babies with a soothing, low-pitched warble.

Time was, some students questioned Andrews’ leadership because she is single and childless.

But not anymore.

“They know I care,” says Andrews, who holds a master’s degree in education. “Some girls will ask why I don’t have any children. And I look at them and and joke about it. I say ‘Right, after seeing what you and your baby go though, I want to run home and have one of my own?’ ”

*

While they have seen numerous celebrities, the Detroit students are more curious about Southern California’s varied flora, the street life of South-Central Los Angeles and the proliferation of homeless.

“That, to be sure, has opened some eyes,” Andrews said. “Many have never seen homeless people sleeping on sidewalks and in doorways. I think it made them appreciate just what they have.”

On Monday, this traveling road show of working Moms and their babies will fly home to Detroit. With their horizons expanded, many say they dream of doing the same for their children.

Until then, they’ll be hard at work. Hammering. Sawing. Sweating.

“What these girls are doing--the drywalling work, building the structure itself, is the hardest thing you can do in construction work,” said Bryan Bossand, who is supervising a Habitat for Humanity project of eight apartments in Pacoima.

“They’re out here climbing scaffolds, asking questions. It took me three days to learn they were all mothers. Man, was I impressed.”

Jamika Davis, 16, explains the thrill of creating something that wasn’t there before.

“It’s satisfying to help somebody who didn’t have a house,” she says, holding her 22-month-old son, Jeremy.

“It’s the feeling of ‘Hey, I made a difference.’ ”