More than 91 percent of Washington's elementary school students were promoted last month, a sharp increase over the past two years, since uniform promotion standards were first used citywide, school officials announced yesterday.

The passing rate was even slightly higher than it had been before the uniform policy went into effect in 1981.

Associate Superintendent James T. Guines said the drop in the percentage of students held back from promotion this year--from 16.3 percent for grades one to six in the spring of 1982 to just 8.5 percent last month--confirms the academic gains the school system has made under its competency-based curriculum (CBC) during the past five years.

Guines added that the high promotion rate also is in line with the scores on standardized tests released last month, which continue to show significant improvement.

"The whole CBC is taking hold," said Guines, who led the development of the step-by-step teaching plan under former school superintendent Vincent E. Reed. "We've been saying for years that all our children can learn if we define things well and train our teachers to teach them. Now we have youngsters who have been in the program all the way to fifth grade. We can see that they've been brought up to those standards."

Under the promotion policy, called the Student Progress Plan, elementary students are promoted or held back at the end of both the fall and spring semesters. For students to pass, teachers must certify they have mastered at least 70 percent of a checklist of required grade-level skills in either reading or math.

School officials said yesterday they did not know how many of the 33,275 students promoted last month had mastered the skills in only one area. Those youngsters are placed in "transitional" classes and are given special tutoring and after-school work in the area in which they did not reach mastery.

When the uniform promotion policy first went into effect in grades one to three in January 1981, nearly a third of the students involved were left back, including 39 percent of third graders.

A year later, when grades four to six were added, the failure rate in those grades was even higher, including 44 percent left back in the first semester of grade five.

However, the retention rate in every grade has steadily declined each semester. For both third and fifth graders it is now just 8.1 percent, according to the new report. Among sixth graders it has dropped to just 3.5 percent, compared to 13.3 percent in the spring of 1982.

The retention rate now is highest for first graders, about 14 percent, but this is slightly less than the rates of 15 and 16 percent recorded during the late 1970s. Guines said that before the uniform promotion policy went into effect, the average promotion rate for the six elementary grades was about 90 percent, having dropped somewhat from a high level of "social promotions," based on age rather than achievement, during the previous decade.

This year the number of specific objectives required for promotion in each grade was cut about in half, Guines said, to reduce paperwork. Mentally retarded students and those with limited ability to speak English were specifically excluded from the promotion requirements. But Guines said the academic standards of the requirements had not been lowered, and he was certain that the changes did not mean that "social promotions" were returning. "If you put these lower retention rates together with our higher standardized test scores, you see that the two go with each other hand in glove," Guines said. "The whole system is doing much better."

Superintendent Floretta D. McKenzie has proposed returning to year-to-year promotions, which has met some opposition on the school board. Guines stressed that dropping the mid-year promotion policy would not change the skills required in each grade, but would make the standards easier to administer. burban Slowdown