Starting next month, nearly 3,000 top federal managers and exxecutives here will go back to school to learn a complex new grading system they will use to rate subordinates for purposes of pay raises, promotions or dismissal.

Uncle sam is gradually phasing out a decades-old system which ranked one million white collar government workers annually as either "unsatisfactory," "satisgactory" or "outstanding." Federal officials say the system has been too vague, and a flop since 98 percent of all workers almost automatically receive "satisfactory" ratings. That entitled workers to regular pay raises within their grades but gave little evidence as to how well they were actually performing their jobs, officials say.

The sessions will consist of oneday seminars here, and will be taken by thousands of managers and supervisors in the field, over the next three months. They are to prepare the bosses for the new, more detailed rating system which will set performance standards for every job in government and establish "critical elements" within each job that can make or break a worker.

Federal officials say the new standards will be "more objective of a manager of employe. The critical elements set for each job will determine if the employe or manager being rated is doing important tasks satisfactorily, or if he or she may be a candidate for dismissal.

Many federal workers are nervous about the now system. Under the civil service reform law, it must be operating for every job in every government agency by October, 1981. Some agencies, such as the Environmental Protection Agency, are alresdy well on the way to setting standards and critical elements for jobs.

Each agency will have authority to set staneards subject to approval from the Office of Personnel Management (OPM). The training sessions for Grade 15 through 18 managers and executoves here will be held at the OPM.

Standarda and the so-called critical elements are related, but different. Standards, for example, might include "timeliness" in performing a job, "quality" of work, and "quantity" of production expected. It could, in jobs requiring meeting or dealing with the public, also include "courtesy."

The "critical elements" of a job would be factors that agencies decide are of such importance that failure to meet any one of them would outweigh sarisfactory or even outstanding performance in all other areas. An example (mine, therefore not too scientific) would be an employe who helped taxpauers with information, or a Social Security worker dealing with the public. It could work like this:

The worker might have an outstanding production recoord, and be tops in supplying accurate, detailed information to the public. But if the employe flunked a critical element -- perhaps he or she was unnecessarily shout-tempered with customers -- that employe could be subject to dismissal.

Federal officials say the new rating system isn't as tough as it sounds -- particularly in the beginning. First, they say, supervisors must communicate to subordinates what the job standards and critical elements of the work are. The employe must agree that he or she knows and understands them. In phase two, officials say, managers must put the emphasis on helping employes correct defects. The hard-ball playing, with firing the maximum penalty, will come once the system is working and everybody understands the rules.

Some agencies may have the new rating system working by this year. Most will have it in operation by 1980 and it is then that the new guidelines will begin to make their impact on production, service and, for civil servants, on raises and promotions.