It seems like a simple proposition: The humber of public school students is declining. Therefore, school costs should drop.

That argument is being made strongly this year by D.C. Mayor Marion Barry, Virginia Gov. John N. Dalton and other area politicians.

But school officials say their reasoning is flawed by double-digit inflation. Budget cuts are next to impossible to make, they contend, without reducing quality.

Politicians and their budget analysts challenge this assertion; citing the general decline in academic test scores during the past decade that also saw the size of school staffs soar.

"I'm not convinced that money is the only answer to our school problems," Barry declared, when he proposed paring the school budget by 4 percent last fall. "It's a tough time. Everybody has to take their share of sacrifices." f

"There's no way we could do that," replied former school board president Minnie S. Woodson, "without hurting children."

In any event, substantial cutbacks probably can't be made without changing what schools so as well as shrinking them. The cutting is bound to be more painful than the growth that preceded it, and considerably more difficult to accomplish.

For example:

Salaries account for about 85 percent of school costs, and substantial budget cuts can only be made by reducing school staff. But because of seniority rules, the effect of trimming personnel is to sharply increase the average age and experience of teachers, thus driving up average salary costs and freezing our lower paid newcomers.

In Prince George's County this already is happening. Since last July, the school system there has hired less than 200 teachers for a work force of about 6,700. Since 1975, the average experience of its teachers has risen from eight years to 12.

"We just have the same people around," one administrator said privately, "getting older and older."

Although closing schools with declining enrollments can save money, it usually incrases the distance students must travel to get to school. That forces more students to ride school buses at a time when gasoline costs are soaring.

Also, in several suburban counties, the number of students in the sprawling low-density areas outside the Capital Beltway is increasing, while those in older, more heavily populated areas has declined.

Thus, in Montgomery County, even though enrollment has dropped 18 percent since 1970, the number of students, riding buses has increased slightly.

Also, buses must be used to transport more handicapped students. As programs for them expand under federal and state mandates, the staff of drivers and bus aides has grown. The Montgomery County school system, with 102,000 students, now has 586 drivers, aides, mechanics and bus supervisors, compared to 444 persons doing these jobs when enrollment was 24,000 higher.

During the same seven years -- 1973 to 1980 -- the price of gasoline has tripled.

Other energy costs, in addition to gasoline, also have increased greatly.

In Fairfax, for example, the cost of utilities -- including heating oil, naturural gas, electricity, telephone, and sewer service -- jumped from $3.3 million in 1972 to an expected $13.8 million this year, even though enrollment has dropped by almost 8,000 students. The utility increase this year alone is expected to be almost 30 percent, or about $3.2 million.

Similar steep rises have occurred throughout the area, mostly because of the cost of fuel.

However, many of the new buildings -- planned in the 1960s and built in the 1970s -- are considerably more expensive to run than older structures. With large open spaces and few windows, they require sophisticated heating and air-conditioning equipment, and run up large bills for electricity and oil.

They also need larger crews of custodians and well-trained, well-paid technicians to maintain them. Montgomery County schools now employ 18 workers full-time to service and repair air-conditioning units.

"When all these things were put together, you could run all this equipment forever for almost nothing," said Ken Hill, the budget director for Montgomery schools. "Now OPEC and Iran have come along. We still have all these things, and we have to make them work."

During the past decade, school programs have changed as well as school buildings, making budget-cutting more difficult.

Efforts to individualize instruction, particularly in elementary schools, rather than teaching large groups, have spawned a substantial cadre of well-paid teacher specialists and aides.

In many cases, texts have been replaced by "consumable" workbooks that students fill in once and discard. Children sitting near each other move ahead at different speeds with non-professional aides helping check their work and special teachers in reading and math giving attention to those who lag behind.

Montgomery has about 1,000 "instructional aides." Fairfax has more than 1,100 "special teachers," in addition to about 5,300 with regular classes.

Throughout the area, average class size has dropped only one or two students since the early 1970s, while the number of teachers for each 1,000 children has increased by 20 to 25 percent, largely because of the specialists. c

In high schools, costs have gone up with the growth of electives. There also has been a major expansion of vocational programs that need costly equipment and small classes.

"When you have the money, you go to innovation as the salvation," said Ransellear Shorter, acting budget director for Washington's public schools. "Now when you don't have so much money, you just have to retrench. But it's difficult to go back."

One other costly change during the decade has been the growth of "special eduction" programs for the handicapped. Because of federal law, budget-cutting in this area is almost impossible, even though the cost per student is generally three times greater than in regular classes.

According to a study by Thomas Muller of the Urban Institute, the number of handicapped children in the Alexandria school system alone rose by 32 percent from 1972 to last year, while total enrollment fell by more than a quarter. In addition, the school board there paid $1.25 million in tuition for 209 handicapped youngsters to attend private school last year, compared to the 46 children it sent to such schools in 1973.

In Fairfax County, Muller's data shows, special education rose from 3 percent of the budget in 1973 to 9 percent last year. Muller calculated that federal requirements for the handicapped added $12.1 million to Fairfax's school spending that was not off-set by federal or state aid.

Even though teacher's salaries have increased, they have fallen well behind inflation, and this has built up pressure for major pay hikes. In Montgomery, where teachers' pay rose only 5 percent annually for the past two years, the teachers union is asking for a 15 percent increase. In Fairfax, school administrators have proposed a 10.4 percent raise, though this has yet to be approved by the school board and county supervisors.

The highest average salary for teachers in the area is in Washington -- $22,263 for a nine-month school year, about 10 percent greater than Arlington, the highest suburb.

Despite these healthy-sounding figures, teachers have seen their salaries slip during the past decade compared to blue-collar workers, such as coal miners and truck drivers, according to the Department of Labor.

The loss of relative status, which afflicts other white-collar groups as well, has added to the teachers' dissatisfactions.

In its monthy magazine, the Virginia Education Association quoted one member: "The man who runs the bulldozer at our country dump earns more than a first-year teacher."

Even with the difficulties in cutting back, the growth of school spending already has slowed, particularly in Prince George's, Montgomery and Arlington.

From 1970 to 1977, spending per student rose 4 to 7 percent a year throughout the area, even with inflation taken into account. During the past three years, the real gain has been just 1 or 2 percent.

As a share of local government spending, public school costs dropped from 43.5 percent in 1970 to 37.2 percent in 1978, according to Philip M. Dearborn, vice president of the Center for Municipal and Metropolitan Research.

"School spending still went up pretty sharply -- about 95 percent," Dearborn said. "That's not exactly great restraint, but it's not going up as much as a lot of things, particularly Metro."