Like skeptical but attentive school children, a few dozen of the men and women who manage and budget various federal agencies sat in a downtown Civil Service training room earlier this week, raising their hands to press an instructor for clearer explanations of things like "decision units" and "output indicators."

Scenes like that are proliferating around Washington in the wake of President Carter's decision to make zero-based budgeting (ZBB) the key vehicle for government planning. Federal officials are signing up in record numbers for training courses that offer to teach them what it is.

What these officials think of the ZBB concept is crucial, experts say, because if they don't like its looks, or if they feel threatened by it, they could drown it in a tidal wave of their own paper as only bureaucrats know how to do.

Administration spokesmen who are "selling" the concept for Carter, as well as some of the federal officials, say the bureaucratic response to ZBB so far seems favorable, or at worst neutral.

Other say the bureaucrats' interest in zero-based budgeting is inspired, at the moment, by a natural desire to keep up with the competition, learn how to defend their own turf and as one budget expert put it, "to avoid looking like boobs" when the time comes to fill out the first new forms.

As President Carter himself explained it, zero-based budgeting "strips down the budget each year to zero and starts from scratch, requiring every (activity) that spends the taxpayer's money to rejustify itself annually."

In the past, managers had to justify only requests for increases; the existing programs were not routinely questioned. The new approach will require managers to categorize their planning in new ways.

However they may feel privately about ZBB, federal officials "have no choice but to follow the President's mandate" and implement it at once, observed a Civil Service financial training specialist.

In any case, the Civil Service Commission and the Department of Agriculture graduate school both report they were unprepared for the unusually heavy response from top federal managers to their courses in the subject. The courses range from a series of two-day sessions ($150 each) at Civil Service to 10 two-hour evening sessions ($38) at Agriculture's graduate school, which is also planning to start day sessions.

Private enterprise has mobilized as well, as one federal training official put it caustically, "to help out the poor old government."

Xerox Corp., among other private concerns, has weighed in with "Dear Government Agency Executive" letters announcing a weekend workshop, April 26-27, at its $70 million international training complex set on, as the letters put it, a 2,265-acre site "among the green Virginia hills near the historic town of Leesburg."

The two-day course costs $300 or more persons, Room and board is $40 extra. (Under federal employee training programs, each agency pays for its own personnel who attend.)

The workshop leader will be Peter Phyrr, the man credited with originating the whole concept and introducing it to President Carter when he was Governor of Georgia. Pyhrr, now a vice president of Alpha Wire Corp. in New Jersey, is in demand of a lecturer. The book he wrote five years ago on the subject (Zero-Base Budgeting: A Practical Management Tool for Evaluating Expenses, Wiley-Interscience, $19.25) recently became probably the first manual on budgets and planning to make Washington's non-fiction best-seller list.

"The phone here has been rining off the hook," said Sharon Paprzycki, a national account manager at Xerox Learning Systems. "Compared to other local seminars we've offered, the response is phenomenal."

She paused to answer one of the ringing phones and then said, "That was two generals signing up. That makes three or four generals so far."

She said around 200 people are expected for the April workshop, with another tentatively planned for May.

Both Georgetown and George Washington universities have started offering courses in the subject.

"We've turned people away from every course," said Debbie Tabas, of the Georgetown School of Continuing Education.

She said most of those who sign up are government employees, attending at government expense, with a few people from private business.The courses range from one day ($125) to four days ($375) and include instructors "who helped implement the technique for Carter in Georgia."

At George Washington University, a recent 2 1/2-day seminar ($410) included an even mix of federal, state and city officials and private sector managers, according to a spokesman.

A top congressional budget expert said the government manager confronted with zero base budgeting seems to be nervous and defensive for two seasons.

"First, he doesn't know how to fill out the forms. he has learned to fill them out a certain way over hundreds of years. Second, he doesn't know what goes on the form, or in other words, how to defend his turf. He wants to know how the output is going to be used . . ."

Federal officials are generally reluitant to give their opinions on zero-base budgeting. Most of those interviewed said they would "wait and see." They did not want to be identifies.

One manager taking a Civil Service two-day introductory course said, "One thing I have learned in this class is that zero-base budgeting really has nothing to do with zero. It has to do with the 'minimum acceptable.'"

A training leader later said that impression was not necessarily accurate, however, ZBB "might involve zero in some cases, where a program is considered to be of such minimal value that it is abolished. Others, of course, such as the interest on the national debt, cannot lawfully be abolished."

A senior Army fudget analyst taking the course said, "Basically, people are saying, 'Be more efficient.' This is just a mechanism for doing that - but you have to tailor it to your own agency's needs."

In its march from the Texas Instrument Co. where Pyhrr introduced it in 1969, across Georgia, where Carter implemented it in 1972, and into a number of other major corporations and state and local governments, zero-base budgeting has been controversial but also in many cases successful, according to experts. Among other things, critics claim it will require bureaucrats to crank out more paper even than they do now. Supporters say ZBB will give planners an unprecedented understanding of how the bureaucracy works and what to do about it.

Some observers have warned President Carter that he is pushing the ZBB program too far, too fast for the bureaucracy to absorb it.

They point out that it is the descendant of other well-intended efforts to look at federal programs with better understanding, such as Management by Objective and Work Measurement.

"In 1967, I went around with Charlie Schultze (then director of the Bureau of the Budget) trying to sell something with the godawful initials of PPBS - Planning, Programming and Budgeting System," said Samuel M. Cohn. Now a vice-president of Robert R. Nathan Associates, he was formerly the top career official in the Bureau of the Budget.

That system was excellent, sensible and ought to be done, but I think it fell on its face because (the Johnson administration) tried to push it off too fast, on every agency at once. It was too advanced for the clerks who had to do the work. And the top managers, the appointees brought in from outside, in many cases leave government just about the time they've learned their way around. I think, zero-base budgeting has the same problems . . . I think Carter should slow-down, and pick his spots."

As the Congressional budget spokesman put it, "The budget bureaucracy in this town is good. If they're frightened into a defense, they'll send you paper like you've never seen - and it won't mean anything. If the Redskins only had their defense . . ."

According to James I. McIntyre, deputy director of the Office of Management and Budget (OMB), the administration "does not intend to unilaterally thrust a budget process on the agencies.

"We have been meeting with key budget people from the various agencies and with Congressional budget people . . . We plan to let them help us develop the process that will be used."

He said the administration also will see to it that there is training available for those who need it.

McIntyre, who implemented zero-based budgeting for Carter in Georgia, said that the concept is really a simple one, "though a lot of folks who don't understand it make it out to be complicated. Some people want it to seem complicated, I guess."

As for the current boom in zero-base budgeting instruction, he said, "I think it's commendable that federal employees want to learn something about it."

He cautioned them not to "make a lot of decisions" until OMB issues its final guidelines to the agencies on the process to be used in developing the fiscal 1979 budget.

The guidelines have been submitted to the President and are expected to be distributed to the agencies, officials said, in the next few days.