With a declining enrollment and a major step forward for a new curriculum, Washington's public schools are scheduled to open next Thursday for the fall term.

Officials expect about 115,000 students, down 5,000 from a year ago and the smallest number to attend D. C. schools since 1958.

The number of schools open also will be less - six fewer than last year because of a school board vote in June to close them. But the system still has a surplus of about 20,000 seats and faces considerable pressure to keep its budget down by closing more buildings.

The new highly structured curriculum, which Superitendent Vincent Reed calls competency-based, will be used for the first time on an experimental basis in 30 of the city's 168 regular schools to teach reading, writing, and mathematics.

Another 26 schools will test new lessons and objectives in other subjects.

Teachers in other D.C. elementary schools will get new curriculum guides showing the topics they should teach and the order in which to teach them but without lesson plans and tests.

Eventually, Reed said, the competency-based curriculum will spell out in step-by-step detail how all major subjects will be taught throughout the school system. The new curriculum will include a required series of tests that students will have to pass before they can move ahead to new work.

Last April, Reed announced that the full program would not be ready to put in all schools by fall, as he had originally expected.

"Now we should have a curriculum guide that really works," Associate Superitendent James T. Guines said last week. "It's not completely bug-free but it's much better."

Reed said that the beginning of fall classes should be "real smooth. We don't anticipate any additional problems," the superitendent said, "just the usual ones."

However, contract negotiations between the school board and the Washington Teachers Union have broken down and the union's dues check-off has been suspended. Unless negotiations resume, union leaders said teachers will "work to the rule," refusing to do any school-related work, such as grading papers or preparing new lessons, outside their work hours at school.

The contract covers teachers work rules and working conditions. Salaries, which are in the highest in the metropolitan area, are set separately by the D. C. City Council.

In the past negotiations the school board has sought unsuccessfully to lengthen the teacher's 6 1/2-hour work day, but current bargaining has not gone beyond preliminaries. The board is pressing for an 80-hour limit on the amount of time that teachers union negotiators can be away from their classrooms to attend bargaining sessions without losing pay. In the past, there has been no limit.

The teachers' contract expired July 25.

Along with declining enrollment, the school system will have fewer teachers - about 300 fewer than a year ago. Although retirement and resignations accounted for most of the reductions, about 100 teachers with low seniority were laid off.

Officials said average class size will be the same as last year - 25 students in elementary schools and 27 in junior and senior high schools.

The D. C. school system will continue to have enrolled its most famous pupil - President Carter's 10-year-old daughter Amy, but Amy is switching to a new school. Instead of Stevens Elementary near the White House, where she attended fourth and fifth grades, Amy will be starting sixth grade at Hardy Middle School on Foxhall Road, NW.

Schools officials granted her request for an out-of-zone transfer. Reed said hundreds of such transfers are allowed every year.

Children who will be 5 years old by the end of this year are eligible for all-day kindergarten at most elementary schools. Those who will be four years old by Dec. 31 may register for half-day pre-kindergarten programs, also offered in many of the city schools. Those who will be six years old may register for first grade this fall.

Anyone between the ages of seven and 16 must attend school. A birth certificate, proof of residence and immunization against measles, rubella, mumps, polio, tetanus and diptheria all must be presented in order to enter school for the first time. Except for rubella, a history of having had these diseases is acceptable instead of immunization.

Immunization may be obtained from the area's free health clinics. For further information, call 673-6700.

Parents should expect their children's first report cards to arrive shortly after the end of the first quarter on Nov. 17. The second advisor period will end Jan. 26, 1979, the third on April 13 and the final period on June 14, the last day of school.