It was the first day of school at Lake Ridge Elementary School in Prince William County and second-grader Theo Watson, 7, clutching a new "Return of the Jedi" folder, dashed through the school's front hall toward a row of classrooms.

"Whoa, Theo. Let's at least find the right classroom," his father, Michael Watson, called after him, adding in an aside: "I don't know what's going on. Yesterday, he was miserable."

Meanwhile, Theo's sister Suzanne, 5, dressed in a pink miniskirt and about to enter the first grade, was holding on tightly to her father's hand, decidedly more hesitant than her brother about the start of school.

For the Watson children and their schoolmates, there was a sense of excitement and uncertainty at Lake Ridge Elementary School, which opened its doors for the first time yesterday to 576 kindergarten-through-fifth-grade students.

Lake Ridge is one of only four new public schools to open in the Washington metropolitan area this fall. The other three, all elementary schools in Fairfax County, are Cherry Run School in Burke, Newington Forest in Springfield and Oak Hill in Herndon.

"A new school these days is something special," said Alton C. Hlavin, an assistant superintendent of Fairfax County schools, who oversaw the planning of the three new Fairfax schools. "In the 1950s and '60s there were new schools popping up all over. Now there are fewer so they get more attention, more planning. A new school becomes a special event."

The new Lake Ridge Elementary school is located on Hedges Run Road in Dale City, located in the fast-growing eastern end of Prince William County. The county's population has doubled in the last decade, leading to increasing demands from young families for new schools.

In Fairfax, the new schools have been built in once-rural areas where subdivisions of new homes are drawing families with a school-age children. Meanwhile, in older and more established areas of the county, where the number of school-age children has been declining, schools are being closed.

At Lake Ridge yesterday, excited children, swinging colorful lunch boxes bearing legends of such summer movie favorites as "Dark Crystal" and "Krull," trailed like happy broods of ducklings behind parents who had volunteered to lead them around their new school. The classrooms were spotless, the books brand new and the halls free of the familiar smells of chalk and worn sneakers that seem a part of more established institutions of learning.

"This is the third new school I've opened and it's just as exciting every time," said veteran Prince William school principal Dean Kilby, who spent the morning greeting school buses and hugging crying children. "Most of my staff of 27 teachers asked for transfers from other county schools to this school because they wanted to be part of molding something new.

"It takes time for a school to develop a personality, but new schools are so rare today they get a lot of help early on in terms of setting goals."

Parent volunteer Pat De Main, whose son will attend third grade at Lake Ridge Elementary, said that one goal that the parents set for the school at a special meeting last week was to become more involved in the school themselves.

"Parents fought long and hard for this school and we plan to keep interested in what goes on," she said. "Lake Ridge is growing so quickly; this school is important in helping us become a community."

De Main said that parents in the Lake Ridge neigborhood wanted a school closer to their homes and Prince William school official Charles Wildman says the new school has helped to achieve this.

"The school is right there in the community, so they can walk," said Wildman of the new Lake Ridge students. "We often are taking them from a busing situation and putting them in a closer school."

Newly constructed schools look different from those built in the 1950's and 1960's because tighter budgets restrict ornamental trimmings and new attitudes toward learning call for a less institutional atmosphere.

"Architecturally, every school used to be an egg carton with a long hallway and classrooms lined up on each side," said Hlavin. "Some were very attractive on the outside but they are cold and institutional on the inside."

Schools built this year, like Lake Ridge, with costs in mind, were planned by educators who prefer classrooms in clusters surrounding a media center, said Hlavin.

Parents and community residents were consulted before the new schools were designed, said Hlavin and Wildman. In the case of Lake Ridge, the $3.5 million, squat modular school was designed with a separate entrance for the auditorium so that community groups can meet there in the evenings without walking through the school's hallways.

"Inside, the schools have more depth visually. They are an exciting place to learn," Hlavin said.

Spokesmen in other counties and cities in the District area said they have no plans to build new schools in the immediate future. Instead, most school administrators continue to grapple with possible school closings prompted by declining enrollments and budget restraints.

At Lake Ridge, all 576 children were in their classrooms by 9:15 a.m. The school has a capacity of 650 students, but planners originally had thought that only 407 would attend in its first year and that the school would fill up to capacity as new houses were built in Lake Ridge.

"At this rate, we could very well see another new school in this area," said Superintendent Richard Johnson, who was on hand for Lake Ridge's first day.

Brandi Haines, 8, the daughter of a lieutenant commander in the U.S. Navy, moved here from San Diego with her parents just a week ago.

"Everything is so nice," she said as she stood in the Lake Ridge lobby, swinging a red book bag. And, she added: "It's nice not being the only new one here."