IT is noon on a crisp autumn Saturday, and at Register 3 in Roche's supermarket, 18-year-old Betty Shaw has just begun her workday. ``Hi, how are you?'' she says, greeting a customer. Then, picking up a loaf of bread, she sweeps it across an electronic scanner that beeps as it reads the universal price code. Bread sweep-beep, cheese sweep-beep, chicken sweep-beep, milk sweep-beep -- the rhythmic motion becomes almost robotic as item after item passes through her practiced hands.
``That's $14.12,'' she tells the shopper. A teen-age boy bags the woman's groceries and carries them out, and another customer steps up to the register.
For the next seven hours Betty, a high school senior, will repeat this scenario dozens of times. During her 2 years here, work has become as routine a part of her life as school, and she is philosophical about what she does.
``It's not a terrible job,'' she says during an interview on a day off. ``I used to be sick of it, but now I get more money because I've been there for such a long time. I relate well to the customers and the people I work with, so it doesn't really bother me.''
Ask any random sampling of teen-agers about their jobs and the responses are likely to be similar -- occasionally enthusiastic, but more often matter-of-fact: ``Yeah, sure, it's OK.'' But they like the money, the activity kills time, and a job provides a sort of independence -- motivating factors that have put a majority of high school juniors and seniors into the work force. Teen-agers, in fact, now form the backbone of many suburban businesses during evening and weekend hours.
Once largely the province of the needy, teen-age work has become an accepted, even expected rite of passage for middle- and upper-middle-class students as well. School can teach the three R's, the reasoning goes, but it takes a job to teach the fourth R: responsibility.
Now the rite of passage is beginning to get so-so reviews from adults as well as adolescents, and the revisionists have arrived. In a new book, ``When Teenagers Work: The Psychological and Social Costs of Adolescent Employment'' (Basic Books, $17.95), Ellen Greenberger and Laurence Steinberg argue that teen-age employment is not always the benign experience it has been taken for.
Jobs, they say, often acquire precedence over school, interfering with homework and keeping students from extracurricular activities. Even the family dinner hour, an increasingly endangered species, is affected as teens race off to scoop ice cream, flip burgers, pump gas, and stock shelves. If paid work becomes too dominant, the authors caution, adolescents ``may be bypassing the equally rigorous, but unpaid, work of growing up -- work that requires exploration, experimentation, and introspection.''
In addition, working teens may be susceptible to what one researcher calls ``premature affluence.'' No longer required to contribute part of their earnings to family coffers, middle- and upper-middle-class students often spend their paychecks on highly disposable items such as records, cosmetics, and faddish clothes. Rather than learning the value of a dollar, they learn the habits of consumerism.
But the main problem, according to Ms. Greenberger and Mr. Steinberger, lies in the kinds of jobs now available to young people. Unlike teen-agers of earlier generations who worked in crafts, factory, and farm positions, today's adolescents are overwhelmingly clustered in retail and service jobs. This ``new adolescent workplace,'' the authors maintain, offers little preparation for adult occupations.
Another revisionist, Amitai Etzioni, a professor at George Washington University and the father of three teen-age sons, concurs. Writing recently in the Washington Post under the headline ``The Fast-Food Factories: McJobs are Bad for Kids,'' he states: ``These are breeding grounds for robots working for yesterday's assembly lines, not tomorrow's high-tech posts.''
That viewpoint remains unpopular in many circles. Mr. Etzioni's article drew a strong letter of protest several days later from a former McDonald's employee, who listed the ``many useful skills'' he learned during two years of employment there -- among them ``patience, responsibility, teamwork, leadership, effective methods of managing people.''
Jeffrey Newman, executive director of the National Child Labor Committee in New York, adds, ``Work teaches kids a lot of things that are part of the ethics and values of our society. You learn about lots of people and you learn about yourself.''
And teen-agers themselves, for all their professed neutrality about work, offer persuasive arguments in defense of their jobs.
``I'm working with different people,'' says Mary Elizabeth Dailey, a high school junior here in Needham who puts in 15 hours a week in a fabric store. ``One is really easy-going. The other isn't. I've been learning how to deal with each one, and with customers. Some are really picky.''
Students also talk about that fourth R, responsibility. ``My boss trusts me with keys to the buildings,'' says Mark Messias, a 16-year-old who cleans offices in a Needham industrial park three times a week. ``I know I have to do the work or he'll lose his contract.''
His mother, Elaine Messias, sees other value in teen employment: ``My experience has been that when kids don't work, they have a lot of time on their hands for watching `General Hospital,' hanging around on the corner in the afternoon, and doing other unproductive things. And they don't have the independence to buy a gift for a family member without having to ask for money. Generally it's good for them as long as it doesn't take over. It gives a sense of self-worth.''
But, she adds, ``It's important for me as a parent to emphasize that grades are primary, work is secondary.''
That kind of parental involvement is essential, authors Greenberger and Steinberg say. Recognizing that part-time work will continue to be a firmly entrenched fact of adolescent life, and that ``most youth can profit, presumably, from good work experience,'' they urge parents to monitor both the kind and amount of work teen-agers do.
They suggest a ceiling of 15 hours a week for high school sophomores, 20 hours for seniors.
These cautionary notes may be well-timed. The revisionist thinking comes at a period when a shrinking labor pool is creating a shortage of employees for many businesses across the country.
At Roche's supermarket, for example, where Betty Shaw works, help-wanted signs carry a slightly urgent tone: ``Students, mothers, senior citizens. Roche Bros. needs you.'' Other stores and businesses make similar appeals. And a notice on the door of a local dry cleaners advises customers: ``Due to help problems we have to close at 3:30 on Tuesdays and Thursdays. We are sorry for any inconvenience.''
As desperate employers lure new workers with promises of flexible hours and good benefits, youthful applicants will continue to join the work force, eager for money, experience, and a firsthand look at an important part of the adult world.
But the old assumption that all work is automatically a positive experience can no longer be assumed. The new consensus finds it inadequate to say, as one teen-ager summed up, ``It's better than sitting around, doing nothing.''