Rapidly developing Fairfax County has passed Prince George's County and now has the largest public school enrollment in the metropolitan area.

"We're Number One!" boasted Supergram, the weekly newsletter of the Fairfax school system. But being No. 1 in students -- who cost the county a minimum of $1,737 each year to educate -- is bad news for the fiscal planners.

"The statistics are pretty grim," said Myron E. Cole, the school system's chief budget officer, citing not only the new figures on students but the cost of educating them.

Although Fairfax's school population actually declined by about 1,600 students from a year ago, Prince George's County school system lost most -- about 7,000 pupils, or 4 1/2 times the Fairfax figure.

That accounts for Fairfax County's new ranking as number one in enrollment.

On Sept. 10, Fairfax had 128,047 students, or about 4,500 more than Prince George's County, although Pringe George's total population is roughly 63,000 greater.

Montgomery County, with an overall population of slightly less than that of Fairfax, reported 100,878 students.

Despite the decline in total enrollment, Fairfax school officials said there has been a doubling of what is called growth within the system -- when the size of incoming kindergarten classes is compared with outgoing 12th grade classes.

While many area localities have had significant declines in their school populations -- a trend resulting from children of the baby-boom years completing their education -- Fairfax appears to be an exception with important ramifications for budget-makers.

Gov. John Dalton has told Virginia localities that instead of counting on the state to provide more aid, they should be able to reallocate some money they used to spend on schools to other areas,such as mass transit. Dalton has maintained that's possible because of declining school populations.

But preliminary figures indicate that schools in older sections of Fairfax County are not losing students as rapidly as expected and, in some cases, are actually "turning around" and gaining students.

School system planner Gary D. Chevalier said the growth in older sections (Mount Vernon, Lee, Mason and Providence districts and the older part of Springfield) could be cause by younger families with children moving into houses vacated by families whose children have grown up.

If the trend continues, Chevalier said, the school system may have to postpone plans to close under-utilized schools and consolidate their students elsewhere.

Chevalier said, the bullish enrollment figures are due in large part to what he called the "explosive growth" in the Pohick area south of Fairfax City as well as fairly rapid development in the Reston area.

Much of the growth is in single-family houses, which generally have more children than townhouses and apartments.

The September enrollment -- because it included 1,254 more students than had been projected earlier -- forced the school system to hire 67 more teachers and other personnel at a cost of $872,000, according to Cale.

Each new elementary student costs the county $1,737 to educate during the year. An intermediate student (grades 7 and 8) costs $1,969 and a secondary student (grades 9 through 12) $2,248.

With Fairfax facing a 20 percent increase in its share of the Metro bus and rail operations deficit -- from $13.6 million to $20.7 million annually -- there will be increasing pressure on the school system which claims about 50 percent of the county's overall budget -- to absorb cuts as it did last year and earlier.