Loudoun County school officials said Tuesday that an additional 710 new students enrolled within the first 10 days of the school year--fewer than an initial estimate of 1,181 new students.

Although the influx was smaller than expected, it has meant that one new campus--Harper Park Middle School in Leesburg--is already crowded, with 32 more students than it was built to accommodate. Hillside Elementary in Ashburn, which opened last summer, enrolled 185 additional students between Aug. 30 and Sept. 13, bringing its student population up to 702 and closing in on the school's capacity of 800.

By Monday, 28,473 students were attending Loudoun's 45 campuses. School districts must report total enrollments to the state by the end of the month. Sam Adamo, the director of planning and legislative services, said he expects Loudoun's final numbers to approach his estimate of 28,884.

But, for the first time, a new study has found that student enrollment growth may be slowing down.

Preliminary figures from the new school census show that the number of children ages 1 to 4 increased 16.2 percent in the last three years--the lowest rate of growth since 1986, Adamo said.

In the 1996 school census, the number of children in that age group had increased 35.2 percent over the previous four years. "The pace and rate of growth is starting to come down," Adamo said. "It is no longer daunting."

He attributes the trend to aging baby boomers who caused the explosion of birth rates during the late 1980s and early 1990s when they were younger but now are becoming too old to have children. "Nationally, that population is getting older," he said.

The 1999 school census is used to determine how much state tax money will be returned to the county for public education. Fifty census takers knocked on doors in April and May asking residents the number and ages of the children in their households, as well as other relevant planning information.

School officials cautioned that the rate of increase--though smaller than in past years--still signals the need for new school construction. An estimated 22 schools are planned in the next six years to accommodate Loudoun's growing student population.

"I think growth will still be part of our picture for a long time to come," Superintendent Edgar B. Hatrick III told School Board members. "Nothing you see here changes the schools you have to build in the next [few] years."

Voters will be asked on Nov. 2 to approve construction of four of those schools: an elementary school at Lansdowne, a middle school in Countryside, an intermediate school for eighth and ninth grades in western Loudoun and a high school in Leesburg.

The $111 million referendum also includes a proposal to expand Lucketts Elementary and replace the heating and cooling systems at four other campuses. A separate question on the ballot will ask voters to support a $5.3 million addition to the North Street School Administration Building.

To press their case, school officials said Loudoun schools under construction cost less than the average school built in Virginia.

Evan Mohler, an assistant superintendent, said it costs an average of $102.66 per square foot to build an elementary school in Virginia. Little River Elementary, which is under construction in South Riding, costs $89.51 per square foot. Mill Run, which is being built in Broadlands, will cost $100.99, Mohler said.