When Northwestern High School decided more than two years ago to get hundreds of students out of the classrooms and into the community one day a week, there was a lot of skepticism all around.

The concept sounded fine in theory. Influential organization such as the Carnegi Council were calling for changes in the "deadly weekly routine" of the traditional American high school. Reformers were saying that a day or two a week at the local hospital, city hall or bank could develop confidence, curiosity and lifetime skills that weren't available from textbooks.

But would the idea work in practice? Could teachers run a school with scores of students excused on absences every day? Would street-wise city kids in need of money and high school credits sign up for unpaid, no-credit work in community institutions and offices?

When more than 600 of the school's 2,000 students applied last fall, some faculty members wondered if the kids weren't just using the new program to cut classes.

"It was panicky for me at first, I guarantee you," admits Jack Knott, one of two teachers who coordinated the experimental program. "I was constantly worried that they weren't at their sites, and would call to check up on them all the time. But by February or March I realized that the kids were out doing what they were supposed to be doing. So I just relaxed and enjoyed it."

Teaching professional educators to have a little more faith in the students they serve has been just one of the side effects of Maryland's Community-Based Learning and Service (CABLES), a radical innovation financed with federal funds that has just finished its second year at Northwestern and at rural La Plata, Md., High School.

CABLES is yet to be subjected to a tough formal evaluation that would analyze its impact on the persistent problems of absenteeism, school discipline, dropouts and grades.

But it has showed that inner-city students involved in this kind of community service come back from it with a sharper understanding of their education and of the attitudes and skills needed to survive in the adult world.

Dawn Frazer, a Northwestern student who is a gifted piano player, "learned what a teacher has to go through" when she volunteered to give piano lessons to seventh and eighth graders one day a week. "It's not as easy as just sitting in a classroom," she added.

Several volunteers who served as teacher's aides in an elementary school classroom said they had gained a new perspective on their education.

"It made me see that other children had trouble learning, and that I'd better stay with the books myself," said a young woman.

Others became more realistic about their goals after first-hand experience.

"A lot of the kids had ideas of what they wanted, like working with children, or in the health field. Working in a school or in a hospital, some were able to find out this was not for them," Knott says.

Knott says the best student testimony about the program is that a large number of those who are not graduating next year have signed up to participate again next year.

State education officals are sufficiently impressed to suggest that the experiment has at least the potential to change the way that high schools, including those beyond Maryland, serve young people.

"I'd love to see it spread," says State Superintendent of Education David W. Hornbeck, whose office has channeled about $80,000 in federal funds to Northwestern and La Plata to support the CABLES program.

"You don't have to be a great theorist to conclude that one learns by doing," Hornbeck says. "The interesting question is whether the fundamental perspective of the high school might be altered by having a sense of mission beyond just teaching academics. We're interested in the notion that school spirit could be determined by the extent to which kids are reaching out to the community."

Many schools have "work/study" programs requiring a brief stint in an office or hospital. Maryland also has a "work waiver" program that enables high school seniors to drop most of their classes except English so as to hold down a full-time paying job. And vocational programs all include extended work experience.

However, these programs have sometimes been critcized for doing little more than providing pocket money or narrow, technical skills. (One California vocational education program was found to have spent hours teaching students how to bag dry cleaning).

By contrast, Hornbeck insisted that CABLES be an educational program rather than an empolyment service, and that the program become a major part of the school curriculum.

Several school systems balked at trying the experiment because of Hornbeck's ambitious target of 50 percent student participation within three years. Northwestern hopes to reach that next year.

In addition to the service work in schools and hospitals, volunteers worked in a bank, at a newspaper and in various offices. Knott said students were sent out only after it had been made clear to the office accepting them that educational responsibilites were entailed.

At each of the sites, supervisors signed an attendance slip and wrote evaluations of the volunteer at the end of the program. One bust banker spend five minutes briefing a volunteer about the business at the end of each weekly volunteer session.

The CABLES experiment, says Janice Earle of the State Department of Education, is a "back to basics" program under the Maryland Board of Education's definition.

The board said in 1979 that "basic education" must go beyound mastery of traditional skills such as reading and writitng, to include such skills as "knowing community resources," "demonstrating social awareness," "knowing civic responsibilites in a democratic society," "relating personal qualities to employment" and "understanding the nature, structure and requirements of work."

"The truth is that high school is deadly, most of it," she says adding that "something is happening" at Northwestern.

Most of the 20 veterans of the CABLES experiment who congregated after graduation last week to share experiences tended to agree with that assessment, as they had described some of the things they'd learned.

Phyllis Robertson, who had volunteered as a teacher's aide in a third-grade class, was "shocked" to discover that some of her students "didn't know their ABC's."

Last winter Michael Merritt worked at the Fuel Assistance Fund office that provided help for poor families unable to pay their bills. In this job, he said, he had learned "how to listen to problems."

"People brought in their personal problems . . . they just let them spill out. But some of the people were reluctant to ask for help. People came in with a very negative attitude. We had to learn how to make them see the positive side and get the help that was available." Merritt said.

Although many of the volunteers came from tough neighborhoods, may were suprised at some of the realities they confronted as volunteers.

A girl who worked in an outpatient clinic was shocked at the large number of child abuse cases treated there.

The rewards of the volunteer work sometimes are intangible.

Gregory Thompson is a matriculating 10th grader with acting talents. Last year he set up his own acting course for third graders at an elementary school in his neighborhood. The class created a skit centering on the drama of police catching someone selling drugs.

But the payoff for Thompson was in the relationship he developed with the younger children he'd never met before. "When I'm on my way to the bus in the morning those kids recognize me," Thompson says, "they'll yell over to me, 'Hey, you coming Friday?' It makes you feel good."

Some of the unpaid volunteer work has led to summer jobs, in a boutique and in the laboratory of a commercial blood bank, for example.

But Knott stresses that education, not jobs, is the main objective of the program.

"We're pressing them [the students] to define what the educational component of the experience is, not just to pick up a technical skill," Knott says.

Knott, a teacher who specialized in working with handicapped children before joining CABLES, believes the strengthened bond between the community and the school is one of the positive side effects of the program.

"It shows the community that at least a segment of urban youth doesn't want to just hang out on the streets with their radios," he said.

For traditional teachers, the adjustment has not always been easy, Knott acknowledges. Even after two years, not all of Northwestern's teachers are ready to accept a program that dramatically disrupts the usual school routine. Some days a third of the students in a class may be out doing volunteer work.

Since the volunteers are required to make up missed tests and catch up on material covered in class, the program maked for additional work for instructors.

Knott and fellow coordinator Bob Johnson cope with this by trying to get other teachers to join the search for places to send volunteers.

"The program calls for teachers to find new ways of teaching," says Loretta DiGennaro, a coordinator at La Plata High School.

But DiGennaro says the program can be a boon to teachers.

For one thing, classes are smaller on days volunteers are out, she says.

"One kid said he hated to see the CABLES kids come back from their day away," she said. "He said it was the first time he'd ever really had a chance to work with the teacher."