When school opens Tuesday, students returning to Prince George's County public school classrooms will select their desks, find their lockers, learn their teachers' names and find some changes:

They will have 6,578 fewer classmates, if projections prove true. The system is expected to enroll 120,980 students this fall. Although 21 schools have been closed during the last three years, there are still 22,000 empty seats.

One elementary school -- Margaret Brent in New Carrollton -- will not reopen Tuesday. Its student body has been consolidated with New Carrollton Elementary School.

This fall 1,400 more students won't wait at the bus stops, but instead will walk to a neighborhood school. Another 2,300 will be bused fewer miles to schools.

Class sizes and student-teacher ratios will remain the same, but school officials warn that the impact of the tax revenue ceiling commonly called TRIM will be felt in the classroom. School maintenance crews will rebuild chairs and rebind books instead of buying new ones. Students in typing classes will learn on manual models instead of electric typewriters. Report cards will come without envelopes. Music instruments, movie projectors and other equipment will be repaired rather than replaced.

For the first time, ninth graders will attend Eleanor Roosevelt and Duval senior high schools, and Thomas Johnson Junior High will become a "middle school" of seventh and eighth graders. This is the first year of a five-year plan to turn senior highs into four-year institutions and convert most junior highs into middle schools. The plan enables the school system to close 11 junior highs and five senior highs by 1985.

Students will have to carry another nickel if they are going to buy lunch at the cafeteria or if they are going to buy milk. Elementary school lunches now cost 65 cents and in secondary schools they are 70 cents.

Residents will elect six school board members on Nov. 4.

No new schools are being built this year, but four senior high schools are undergoing major renovations or getting additions. At Oxon Hill a new science and technology wing is under construction with completion scheduled for September 1983. Fairmont Heights Senior High will be modernized and sometime this year the student body will be moved to the former Kent Junior High as construction proceeds. Central High School also is being renovated and will get a new science wing and an auditorium. An auditorium and later a vocational wing will go in a Suitland Senior High.

All across Prince George's, as students return to classes next week to begin a new year of study, their surroundings will look much the same as last year.

But school officials say all is not exactly what it seems. Behind the normal classroom scenes, officials are sweating to make fewer dollars go farther. School officials can hardly mention, education without speaking of TRIM, the tax revenue ceiling set by county voters in 1978.

"I frankly worry about how we are going to keep up with inflation, negotiate new labor contracts and meet all our program commitments in light of what we know are going to be limited tax resources," school Superintendent Edward J. Feeney told top administrators Aug. 19 in a state of the schools message. "We will have to call on all our resources of management to deal with the issue."

This year the system received only a 2.8 percent increase over last year's budget, while the cost per student rose 8.5 percent. Yet school officials say a full complement of books and supplies have already been shipped to schools, and classrooms will have plenty of chalk and scissors, past and construction paper, textbooks and dictionaries.

Ironically, the other major crisis facing the system -- a sharply falling student population -- ultimately may be the saving grace.

"To an extent the decline in enrollment is well-timed as regards the potential tight fiscal years we face. It gives us some 'slack in the line' which we can use if we manage creatively," Feeney said.

Though the school system was built for more than 160,000 students at its peak in 1971, enrollments are expected to shrink below 100,000 by the mid-1980s.

"There are 22,000 empty seats in the system right now," said school spokesman Brian J. Porter. "We could close 20 schools over the next four years and still have 22,000 empty seats."

In the last three years officials have closed 20 elementary schools and one junior high. But more closings are inevitable. Last spring the school board adopted a policy reorganizing the system's secondary schools. Each year for the next five years, more senior highs will become four-year institutions taking in the ninth grade, while corresponding junior highs will consolidate into "middle schools" of seventh and eighth graders. Surplus junior and senior high schools will be closed.

In the past, citizen task forces have made recommendations on closing elementary schools, but there is speculation that the board may adopt a similar five-year phase out system for closing elementary schools.

"We're not totally abandoning the task force route," Porter said. "But the thing is just too complex to dump on a group of citizens and ask them to make a decision. That's what they pay us to do in the first place."

While declining enrollments force school closings, they also mean fewer teachers and administrative staff and additional money is saved. As students go back to school next month, there will be 399 fewer professionals in the system. All reductions have been handled through attrition and retirements without laying off any teachers, Porter said, and classroom sizes and student-teacher ratios remain the same: an average of 27 students per class in elementary grades; 50 teachers per 1,000 students in junior highs and 49 teachers per 1,000 students in senior highs.

With fewer students in the system, fewer buses will roll. In addition, the board decided last April more students could walk to school or attend schools closer to home. An extensive study of busing and racial patterns showed the old busing plan set up under federal court order in 1973 was busing students from newly integrated neighborhoods to schools in other integrated neighborhoods. The board ruled that 1,400 students could get off the buses and attend school in their neighborhood. Another 2,300 are being bused fewer miles to school.

Six members of the school board will be elected Nov. 4. Three members -- Susan B. Bieniasz (District 4), Bonnie F. Johns (District 6) and board chairman Jo-Ann T. Bell (District 7) -- are running unopposed.

In District 1, board member Doris A. Eugene, appointed by former county executive Winfield M. Kelly Jr. to replace the retiring Maureen Steinecke, faces Paul D. Duncan of Laurel.

In District 9, former board chairman Norman H. Saunders will compete with Mary M. Touchstone of Camp Springs.

In District 3, longtime board member Chester A. Whiting lost the primary election. Running for the seat are Melinda G. Miles of Mount Rainier and Catherine M. Burch of Silver Spring.

Once board elections are over, the board faces negotiations with all three unions of teachers and classified workers. Negotiations begin in the fall. The two-year contracts expire July 1981. Once again the tight budgets dictated by TRIM and stiff inflation felt by employes are expected to collide in the negotiations.

Despite TRIM, inflation and declining enrollments, Porter said he perceives a stout sense of determination among employes, perhaps brought on by the adversity.

As one school official put it, "TRIM is a reality in our lives, but we're not going to fold up our tents."