For several thousand students in 26 D.C. public schools, September is the kickoff time for a new, highly structured curriculum that officials hope will reverse a long-term slide in academic achievement.

The new program, which Superintendent Vincent Reed calls a competency-based curriculum, spells out in unusual step-by-step detail how major subjects will be taught. It also includes a required series of tests that all students must pass before they can move ahead to new work.

If the new curriculum works out well in the 26 pilot schools, Reed said, it will be used starting in September, 1978, in all of the District's 180 regular public schools - from kindergarten through 12th grade.

But Reed and his staff say they are moving carefully.

"We can't be sure how it will work out," said associate superintendent James T. Guines, who has been in charge of drawing up the new program. "Of course, we think it will do well. But it may not work, or it may not work in certain ways with certain kids.

"That's why this is going to be a research and development year," Guines continued, "a year of real experimentation. A lot of time is going to be spent getting the bugs out of this thing. We hope that will save everybody else a lot of trouble."

When the fall term starts in Washington next Wednesday, there not only will be a new curriculum, but also two big new schools, even though the number of students in the school system is expected to continue to decline.

The expected drop in enrollment, which officials predict will be about 3,500 less than a year ago, would bring the number of public-school students here down to about 122,500.

That would be about 18 per cent below the school system's peak of 149,116 students, reached in the fall of 1969. It also would be the smallest number of students attending D.C. public schools since 1960.In fact, the city public schools now have a smaller enrollment than the area's two largest suburban school systems, Prince George's and Fairfax counties.

Recently the suburban schools have had falling enrollments, too, which is part of a nationwide pattern attributed to fewer births. But the decline in Washington has been sharper and more sustained than elsewhere in the area.

Paradoxically, the enrollment decline has occurred just as the city is nearing the end of the largest school construction program in its history.

Since 1972 the District government has spent about $200 million on 33 new schools and additions. Yet very few old buildings have been closed, and the school system now has a substantial surplus of classroom space.

The two buildings set to open next week - the new Shaw Junior High School at Rhode Island Avenue and 10th Street NW, and Reed Elementary (formerly Morgan) at 18th and California Streets NW - illustrate the situation.

The Reed school, named after the first chairman of Morgan's community board, Bishop Marie Reed, has a capacity of about 1,120 students, but last year the old Morgan only enrolled about 400. This fall its boundaries have been shifted to include about 200 students who had been attending a neighboring school, H.D. Cooke. In addition, the school is adding a seventh grade, but it still expects to be less than two-thirds full.

The new Shaw expects to be somewhat fuller - filling about 80 per cent of its capacity of 1,250. But the school contains a large health suite, a day care center and rooms for community activities, which the school system has no money to operate. As a result, a whole floor of one wing of the new building is being used for administrative offices, including ones for officials preparing the competency-based curriculum.

Also, the new Shaw building, like all the others built recently by the D.C. school system, is designed on the open-spaced plan, with large, carpeted rooms that can hold up to 250 students.

The large rooms were envisioned by their planners as informal arenas, offering students and teachers wide choice and flexibility. But the new curriculum that the school system is planning is highly structured. And Shaw itself has had a reputation for difficult discipline problems.

How well the new curriculum and the old discipline problems will fit into the new open-space arrangement is an unanswered question, not only at Shaw, but also at many other large, new D.C. schools.

Although the drop in enrollment has led to a comfortable surplus of modern buildings, it has been accompanied by an uncomfortable decline in the number of teachers.

Average class size in Washington schools has stayed about the same since 1972 - about 25 pupils per teacher in elementary schools and about 30 per class in junior and senior highs. But the number of teachers employed has dropped by about 10 per cent, to about 7,000 this fall.

Until this fall retirements and resignations have accounted for virtually all of the reduction, and only about 20 employees have been laid off.

D.C. school officials say they hope to avoid any layoffs this year, too, but whether they are able to do so depends on how many teachers retire.

"We're just holding our breath every day," said Claudette Helms, the school system's personnel director, "and hoping more retirements come in."

Because the D.C. school system has done virtually no hiring for the past four years, the average age and experience of its teachers has increased substantially, as has the average salary.

Last winter, Reed reported, 47 per cent of all D.C. teachers had at least 13 years of experience compared to just 28 per cent who had been teaching that long six years ago. About 55 per cent had at least a master's degree or 30 hours of post-graduate credit - and were being paid extra because of it - compared to just 25 per cent earning higher salaries because of post-graduate crdits in 1970.

Any layoffs that must take place would be effective Oct. 1, Helms said, which is the start of the new fiscal year for the whole city government.

Although D.C. teachers' salaries start at $11,045 for the nine-month school year, the average salary last year was $16,817.

Despite their relatively high level of experience, graduate degrees and salaries, D.C. teachers are still relatively young. Last year, Reed said, their average age was 38, and only 19 per cent of them were over age 50.

Thus, it is likely that the same teachers will be in the school system for many years, and any changes in teaching here can only come about through retraining courses and introducing new curriculums, not through attracting new blood.

Last summer about 2,000 D.C. teachers took a week-long course on the new competency-based curriculum. Starting in October, the school system is presenting 24 half-hour programs about it on Channel 26 (WETA), for which teachers can get academic credit.

Along with the new curriculum, the school system is also working on a new way to evaluate teachers, using a list of specific objectives in preparing lessons, teaching different topics and testing students, rather than the more general qualities listed on the old rating forms. This year the new "teacher" appraisal system" will be tried out in 50 schools although it is still unclear what will happen to teachers who consistently get low ratings.

The new competency curriculum is also tied in to a new testing program - giving standardized tests in reading and mathematics to all students in grades 3, 6 and 9, and reporting the results for every school in the system. The tests and the school-by-school reports were discontinued five years ago, except for a 10 per cent sample, amid complaints that they were irrelevant and unfair to blacks, who comprise 96 per cent of the city's public school enrollment.

However, the D.C. school board has asked for the standardized tests as a way of measuring student achievement that parents and the community can understand. Reed, who became superintendent in October, 1975, agrees with them. He said the tests will be reported in terms of the usual national norms, rather that using a set of special big'city norms (where lower achievement yields higher scores),

"We want to compare ourselves with the national standards," associate superintendent Guines declared. "We know we have serious problems, and we're not trying to hide them. But we have a curriculum to do something about them, and we think we are going to get results."

In a few more years, the school board has voted, D.C. high school graduates will be required to pass an exit exam, insuring that they have certain basic skills. When this requirement will go into effect, however, and how difficult the skills will be are still uncertain. But they will surely tie into the competency curriculum, and officials see the year ahead as the first major test of what the new curriculum can achieve.