In an era of declining enrollments, school closings and reductions in teaching staff, the Montgomery County school system opens next week facing more of the same.

Things will be smaller and tighter, continuing the trend that began as the baby boom generation started moving out of the schools and as inflation diluted the purchasing power of government funds.

While there are changes and innovations in the curriculum, the cutbacks forced by declining enrollment and revenue will be the dominant theme. The basics aren't affected, some officials say, but some of the extras are.

For the first time since the enrollment decline began five years ago, some teachers will not be rehired. However, of the 1,000 probationary teachers who received lay-off notices in April, all but 130 will be rehired.

Spring vacation has been scheduled for colder weather in an attempt to save heating costs. Many students who were bused to school last year will be walking this year. Buildings may be dirtier because the custodial staff has been cut back. And classes will be slightly larger, also because of economizing.

"I don't think what we've experienced so far necessarily means a perceptible decline in quality," said school spokesman Kenneth Muir. "But if we continue to get cut in the future as we got cut this year (by the County Council), then I think a decline in quality is a real possibility."

The County Council cut $7 million from the school board's proposed budget last spring. It funded a 6 per cent teacher raise only to the 4.2 per cent level that county employees were getting, and it cut some 206 other positions. To maintain the bargained-for 6 per cent raise, the board had to cut other parts of the budget more.

Muir said superintendent Charles M. Bernardo's goal is to manage the decline in size without impairing quality, "and he thinks he can do that."

The school system also faces controversy over school closings, high school reorganization, expanded desegregation and the busing of private school students. And a new discipline policy goes into effect.

School board members are generally optimistic about the quality of education in Montgomery County this year despite the twin perils of declining enrollment and reduced revenue.

Board president Herbert Benington said budget cuts have hurt only in that "we haven't been able to pursue some of the special programs that we'd like to." But board member Blair Ewing said he worried that budget cuts would hurt because of bigger classes and no improvement in the ratio of counselors or art, music and physical education specialists to students.

Elizabeth Spencer, board vice president, questioned "our ability to attract and hold top-quality teachers, ig given the job insecurity they feel" because of the declining enrollment.

Spencer sees firsthand some of the effects of the school system's belt-tightening, through her daughter, a senior at Seneca Valley High School. Because the school's seventh period has been eliminated, her daughter cannot take a course she could have taken otherwise.

But Spencer emphasized that while there have been cuts, "At the same time we are skimping on this, we are expanding services for special education students. We are looking at the possibility of more vocational-technical opportunities for secondary school students - so it has not at all been skimping."

Programs for both handicapped and gifted students are being expanded. More elementary art, music and reading specialists are being hired. And two new programs designed to insure that students really learn math and language arts are beginning or being expanded.

Also, seven schools in the Takoma Park area begin an experiment this year as "magnet schools" open - with free transportation - to all students in the area. Each of the schools will offer specialized programs ranging from one in which students will speak French throughout most of the school day to one highly structured traditional program. One hundred thirty-seven students have chosen to switch schools this year because of the plan.

In general, however, it is a year of cutbacks and inconveniences.

Among the cuts:

Instruction. Students will face larger classes, fewer options at the large high schools and fewer special offerings. But there will be a few new programs too.

The average class size - 26.5 students - will go up a half student. Many school officials find the trend worrisome, because it means more classes with 30 or more students. The seven-period day that has allowed students at 14 high schools to take more classes is being abandoned at seven high schools with 1,400 or more students.

Special programs from summer school to Head Start (55 classes last year, 45 this year) are being cut back. Junior high interscholastic athletics has been eliminated, over considerable opposition, to save $230,000.

Energy costs. Fuel costs have been a major reason for the school system's budget problems, so the board has cut back on busing and heating. Approximately 7,000 fewer seventh-to-twelfth grade students will be bused to school because buses will pcik up students who live only 1 3/4 miles or more from school, instead of 1 1/2 miles as last year.

Also, spring vacation will be held in late February following Washington's birthday instead of at Easter. School officials expert an adverse reaction when students and parents become aware of the earlier spring vacation.

Not all of the school changes this year involve budget cutbacks. School closings, desegregation plans, the new discipline policy and busing of private school students are among the issues that will be in the minds of both the school board and its constituents this year. Many of these issues are left over from last year.

Among these issues:

School closings. Montgomery County was the local leader in closing schools when the enrollment decline hit, closing 16 schools in the last two years. But Bernardo told the board this summer that it would have to close at least 30 schools, including six to eight secondary schools, by 1982.

Until now, the board has concentrated on closing small schools first, often over vocal opposition from the neighborhoods they serve. But the board plans to develop a new policy this year, while still making decisions about schools that must be closed next June.

High school reorganization. Bernardo has proposed a shift from a junior high to a four-year-high school system. Such a shift would require only one high school closing instead of three by 1962. Junior high schools would become middle schools for grades six through eight, and six of them would be closed by 1968.

Desegregation. The board undertook desegregation of the Bethesda-Chevy Chase area schools last year, and will expand it slightly this year. Gilbert Valdez, director f of quality integrated education, said a survey showed that most people thought last year's major changes "went quite well," and he expected no problems as 240 more students are brought into the plan this year.

Discipline policy. The new policy, adopted last year and effective next week, will provide uniform suspension and other disciplinary practices for certain types of actions. Also, it will tie credit to class attendance so that students who miss 10 per cent of their class time lose credit for the course.

Busing of private school students. Maryland law requires that certain non-public school students be given free bus transportation, but Bernardo and Catholic educators have clashed on how much busing the county must provide for parochial school students. The board must still reconcile the two sides.