And just why does Denver have a 700-acre classroom?

"They think they're out of school, but they're not," the bus driver, Joe Sellers, confides as the fifth-graders rush from his yellow school bus onto the mountain meadow. Learning never stops at the Balarat Outdoor School.

For 10 years more than 100,000 pupils from the Denver public schools have used thid year-around education center 47 miles north-west of the city. Their "classroom" is 700 acres of mountain terrain dotted with forests, streams, large hills, meadows, and a pond.

Named Balarat from an Australian term meaning "good camping ground," the land was given to the Denver schools by an anonymous donor in 1968, under the condition that improvements be made but that the ecological systems be preserved.

This ecology is the focus for the fifth-graders from Traylor Elementary School. They listen to the frogs in the pond, learn that sedum can be a survival food, and discover that the bark of a ponderosa pine smells like vanilla when the sap is running. However, this "classroom" can adapt just as well to a seventh-grade art class or a high school group studying geology. Pupils from preschool through 12th grade use it.

A Balarat teacher, Alice Sato, gathers the fifth-graders around her, then divides them into three teams: one to go with her, one to go with their teacher, Dick Bradley, and one to go with Kurt Hopkins, a high school senior who assists them as part of a Balarat eco-seminar course he is taking. Mrs. Sato's team names themselves the "Aberts," after the tassel-eared Western squirrels.

Walking on paths marked by logs half buried in the ground, the teams follow their leaders. They stop to look at the meandering gopher tunnels, clearly visible when the snow has melted. Mrs. Sato points out a fallen hollow log that is "an apartment house" and "a supermarket" for the inhabiting ants and other insects.

"This is a lot better than junk food," observes one wise fifth-grader, as they sample wild onions and other plants that could be used for survival in the wilderness.

"Every inch of the way there is something to enjoy!" Mrs. Sato exclaims. Although she teaches at Balarat five days a week, she finds that each time she leads a group the "classroom" is a little bit different. One student points out a plant the teacher identifies as "grandfather's beard." Another time the group gathers around a rock that shelters a miniature fern -- a rarity in this arid climate. Then they all touch the trunk of an Aspen tree to feel the white "powder" on its bark.

"Children are wonderful -- so spontaneous. They notice and think about what they see," says Mrs. Sato appreciatively. One of five full-time Balarat teachers, she has been with the program since its inception and helped plan Balarat as an education center. Previously she had had experience both as a biology and elementary schoolteacher.

When it is time to eat the sack lunches each pupil has in a backpack, Mrs. Sato pauses. "What does fragile mean?" she asks. "It means don't be rough with it," a fifth-grader answers. Only after they learn that they must be careful with fragile nature -- so that one careless kick of their hiking boots does not trample hundreds of tiny plants -- are the children allowed onto the hillside.

With the deftness of experience, Mrs. Sato occasionally relaxes the rein of discipline. When the group approaches a snow bank on a north-facing slope, she allows those who want to tumble in the white stuff for a few minutes. Then, excess energies expended, they rejoin the trail. "We keep everything on a positive note. We seldom have to get cross or negative," Mrs. Sato remarks. "There are more discipline problems on the playgrounds than up here."

This three-hour excursion into the wilderness has a solid educational objective. The class teacher, in applying for a Balarat outing, must show how this experience will dovetail with, and enrich, programs started in the regular classroom. If the application is accepted, a filmstrip and field guide are sent to prepare the class of Balarat.

Because the trip from Denver to Balarat and back takes 2 1/2 hours, or more, of the school day, the bus is also a part of the "classroom." On the way up Joe Sellers stops the bus next to a large prairie dog colony where the inhabitants obligingly come out of their holes to observer the big yellow vehicle and its occupants. After the bus moves on, the children are shown a diagram of a prairie dog's underground home, so that they can point out the bedroom, the bathroom, and the back door.

On the trip back to Denver, another dirver, Jim Bustos, points out people who are panning gold in a stream near the Bueno gold mine. The children learn that the mineral belt in Colorado extends from Telluride, on the Rockies' western slope, to Jamestown, the little mountain town they are passing.

About 35 percent of fifth-graders in the Denver public schools get the opportunity to spend three days and two nights at Balarat, where they learn skills like using maps and compass, discover microscopic life in the pond, and observe stars in the clear mountain sky. They stay at Elk Lodge, which was built by the school district, and help serve the family-style meals and clean up , afterward. For many it is the first time they have been away from home.

They do things they never knew they could do -- like "learning to walk down a path in the dark and not be afraid." Those who work with Balarat hope that someday they will be able to expand their overnight facilities so that all Denver fifth-graders will be able to take a three-day excursion.

High school seniors, like Kurt, can spend a semester at Balarat when they enroll in the eco-seminar program, which combines ecological study with teaching younger pupils who come for the one- or three-day excursions. While riding the buses and hiking the trails with them, the eco-seminar students encourage the youngsters to ask questions, indulging their natural curiosity.

Seniors help man a Balarat weather station and send regular reports to the National Weather Service. Eco-seminar students also have rebuilt a stone hut (complete with sod on the roof for insulation), which was part of a pioneer homestead that had lain in ruins on the Balarat site for many years. Now fifth-graders bake biscuits on the old stove inside the hut, as part of their resident experience.