In Montgomery County, where 23 schools have closed since 1972 and parents are bracing for more of the same because of declining enrollment, the community of Bannockburn in Bethesda last year saw a sudden and surprising increase of elementary school students.

The increased enrollment at Bannockburn Elementary School - 10 percent more than official projections - was unusual, but not unique to this affluent Washington suburb, say those who study such trends.

Neighborhoods do turn over, with older people who have grown children moving out and younger families taking their places.

"We could most anticipate it," said Katherine Eisenberger of the American Association of School Administrators. "A community does have a life cycle, does continue and renew itself."

The Washington metropolitan area, according to Eisenberger, is one of the first places in the country where the renewal of suburbs will take place. This is partly because Washington attracts so many professional dual-income families, whose members want to live close to the city, she said.

"The impact of the (turnover) phenomenon is not significant enough to call it a trend, but it is occurring, and it's one school administratos should take heed of."

At Bannockburn Elementary in Bethesda, where enrollment had been steadily declining since 1974, school planners predicted that 318 students would show up for the first day of the 1977-78 school year.

But as last summer wore on, more parents kept coming to register new students, according to Bannockburn principal Daniel Shaheen. "That was not surprising in itself," said Shaheen. "The shock was that at about the time you thought 'It's going to stop,' they just kept on coming."

When school opened there were 350 students, and the enrollment at the one-story brick school on the woodsy cul-de-sac of Dalroy Lane peaked at 363 last February, Shaheen said.

Some growth had been predicted because of a new home development in the area, but no one had expected 25 new families with 49 grade school children to move into the older homes in Bannockburn, as a recent count of students revealed.

Ann Mitrisin, whose daughter Sophie was one of the new sixth-graders last year, said she and her husband John moved to Bannockburn partly because of the reputation of the schools, particularly the junior high school that serves the area. They bought their home on Halbert Road from a couple whose son was grown and about to go to college, Mitrisin said.

Realtor John Y. Harrison said the community, started in the 1950s, has many families with grown-up children. But he has recently noted some younger families moving into the homes that generally sell for upward of $110,000. Harrison said he just sold a home "right behind the school" where an older family moved out and a couple with four youngsters moved in.

The entire Montgomery County school system enrollment has been declining since the early '70s and dropped by about 3 percent to 112,625 in 1977. The projection for the coming school year is 109,198, according to school planner John Daly.

It is, in fact, in the highly populated southern section of the country near Washington where school officials will be studying schools in the next few years with an eye toward consolidations and closings.

It was in the early 1970s that school systems all over the county began to feel the impact of the decline in the nation's birth rate - a decline due in large part to the increasing tendency of young couples to have smaller families than their parents and to have them later in life.

"I think it's a real turnover," said George Grier, a demographic expert whose daughter Suzanne was a sixth-grader last year at Bannockburn. "I don't want to overstate it, but I think that school systems located in or close to cities should look carefully to see they don't move too fast with school closings."

In Fairfax County in 1974, school officials were surprised by a sudden surge in enrollment that for one year broke the downward enrollment slide and brought 1,383 more pupils to the system than they had prepared for. It was chalked up to the same type of community turnover, and it has not been repeated according to a Fairfax schools planning technician.

As in the Bannockburn case, some of the Fairfax increases were in the more affluent parts of the county where conventional wisdom held that homes were too expensive for families with young children.

Eisenberger, who is national director of a demographic project for the school administrators association, says it is true that the turnover usually occurs in communities where houses sell for under $100,000. But Washington, she said, draws so many professionals and dual-income families, that this many account for the difference.

"I think it's a real turnover," said George Grier, a demographic expert whose daughter Suzanne was a sixth-grader last year at Bannockburn. "I don't want to overstate it, but I think that school systems located in or close to cities should look carefully to see they don't move too fast with school closings."

It is, in fact, in highly populated southern section of the county near Washington where school officials will be studying schools the next few years with an eye toward consolidations and closings.

Montgomery County school system enrollment has been declining since the early '70s and dropped by about 3 percent to 112,625 in 1977. The projection for the coming school year in 109,198, according to school planner John Daly.

Principal Shaheen does not want to speculate on the reasons for last year's increse, and the projection for this year is, in fact down again to 342.

"Everywhere they're going down and have been for years," said Shaheen. "We just got a reprieve."