White-collar federal workers would have to get the equivalent of a B on their report cards to get longevity (in-grade) raises under a proposal President Reagan is expected to okay very soon.

Currently 99 of every 100 federal workers get so-called in-grade raises whenever they become eligible for them. All that is required is a "satisfactory" rating, which most people get.

In-grades, which are worth about 3 percent, are separate from the regular October pay adjustments federal white-collar workers are supposed to get each year. President Reagan has said, however, there will be no October raise this year.

Fifteen of the government's 18 white-collar pay grades have 10 longevity steps. There is a spread of approximately 27 percent from bottom to top. An employe who spent his entire career in one grade would under the current system reach the top salary for that grade in about 18 years, providing he got satisfactory ratings.

Workers become eligible for in-grades (provided they have a satisfactory job performance rating) every year for the first three years of service, then every other year for the next six years, and finally every three years until they reach the top of the grade.

The plan Reagan is expected to okay would put all agencies on a five-level performance rating system. (Some now have six, some as few as three, rating levels). Ratings would be either "outstanding, fully satisfactory, satisfactory, acceptable or not acceptable."

Workers rated at least "fully satisfactory" would get in-grade raises. Those rated "satisfactory" would not get them anymore.

Once cleared by the White House the new tougher grading system will be put in the Federal Register for 60 days comment. Then it could be put into effect administratively.