Declining enrollment, which has plagued Prince George's County schools throughout the 1970s, probably will continue until the mid-1980s when school populations are expected to increase slightly, according to a superintendent's report released last week.

The report, prepared by the staff of Superintendent Edward J. Feeney as part of the school board's annual planning and review process, predicts that enrollment for the entire school system, which now is 133,000, will fall to a low of 128,000 by 1980 and then make gradual recoveries in the four years after that, reaching 130,400 by 1984.

As expected, the report does not recommend that any new schools be built, although it does call for spending nearly $109 million in the next seven years to complete about 20 capital improvement projects at various school system facilities.

The school board is expected to close several elementary schools in the coming year to cope with declining enrollments. Feeney reported that there are now 19,000 empty seats in county elementary schools.

For the immediate future, the report predicts that school enrollments will continue to fall.

The report explains: "The decline in school enrollment can be attributed to the decline in the birth rate, the limited housing availability due to high mortgage rates, the status of the national economy and fewer housing starts resulting from the sewer moratorium and slow-growth policies."

But that trend will be reversed, the report said, to a large extent because the county recently rescinded its sewer moratorium. "It can be expected," the report stated, "that allocation of sewer authorizations and the issuance of building permits for new dwelling construction will increase in the near future. As families occupy the anticipated new homes, the rate of decline in enrollment will be reduced followed by a slight increase in enrollment in the early 1980s."

In terms of the elementary grades, school officials are predicting an enrollment drop of 2,500 students - from 66,000 to 63,500 - between now and 1980. The elementary school population then should start to rise at the rate of about 300 to 400 students a year, reaching 64,700 students by 1984.

Similar patterns are predicted for the junior high and high school levels. For junior highs, enrollment is expected to drop from its current level of 34,000 to a low of 32,800 in 1980 and then jump to 33,000 by 1984. For high schools, enrollment is expected to slip from its present level of 33,000 to 31,900 by 1980 and then climb to 32,600 br 1984.

The report also outlines all projected building and capital improvement plans slated for the next seven years.

Much of the expansion program will occur in the southern part of the county because, a school spokesman said, "that's where we expect the most growth."

The most expensive project will be at Suitland High School, where officials are planning a three-phase program: to build a new 1,200-seat auditorium, to renovate the existing facility and to add 18 trade and industry teaching stations. The project, which should be completed by the 1984-85 school year, will costs more than $15 million.

Another costly project is slated for Bladensburg High School, where officials hope to renovate the existing building and add a new 1,200-seat auditorium by the 1984-85 school year. The entire projects is estimated to cost $12.3 million.