The roads and sidewalks to Fairfax County's newest school - South Lakes Secondary School in Reston - were still being paved last week, ceilings were being hung floor tiles laid, electric wires and plumbing still being installed. Huge piles of gravel and dirt were outside and boxes and debris inside.

The fire marshals shook their heads and handed out a "punch list" of things that had to be done before the school could open this week to almost 2,000 new students. "I don't see how they can open," said one marshal. "They'll never make it."

"We'll open," said new principal John Broaddus. "We may be brown bagging because the cafeteria won't be finished . . . and one section of the school won't be ready probably until Christmas, but we'll open."

The whirlwind, photo-finish for the county's newest and most expensive school, at $9.2 million, has kept hundreds of construction workers and most of the new school staff working seven days a week since mid-August.

On Friday, teachers were everywhere - on ladders, standing on desks, on their hands and knees. Many were wearing gloves and dungarees. And most were sporting new blue or green tee shirts that said "Sea Hawks" - the school's new colors and nick-name, chosen in a local newspaper poll of Reston residents.

Outside on the new and freshly mown playing fields, beside the new concrete stadium, practicing the new school cheers and songs, were the "Mighty Sea Hawks" band and drill team. They will strut their stuff for the South Lakes junior varsity football team this fall, the only Junior varsity team in the county that will have musical accompaniment, said band director John LaCava. Bands normally play for senior high varsity football teams but South Lakes, with no 11th and 12th graders, will have no varsity.

The 60-acre South Lakes "campus" soon will be home for more than 4,000 Reston students from kindergarten through high school, one of the largest clusters of schools in Fairfax County. While other, older sections of the county are seeing their schools closed as their student populations decline, the fast-growing Reston area will have four new schools in four years in its southern section alone.

Last fall, the solar-heated Terraset Elementary School opened several hundred yards from South Lakes. Although one of the county's largest elementary school, Terraset quickly became overcrowded and a second school nearby - Sunrise Valley Elementary - is now nearing completion. South Lakes is to be a high school and the county school board only last week approved construction bids for a neighboring intermediate school, which should be completed in two years.

Until the dust settles in 1980, there will be continued musical chairs for Reston students and accompanying identity crises.

Some 350 pupils from Terraset will spend this year in the ground floor of South Lakes High School, until Sunrise Valley School is completed next year.

South Lakes will be a "secondary" school for the next two years, housing 7th-10th grade students transferred from Herndon and Oakton schools, until the new intermediate school is built. Then South Lakes will become a full-fledged high school. The 10th graders at South Lakes this year will thus be in the unique position of being the "senior" class in the school for the next three years.

It is not something all of them are looking forwards to.

"There are no older guys," said drill team treasurer Patti Cayo, a 10th grader who was transferred from Herndon Intermediate school. "There will be no upperclassmen for three years," echoed fellow 10th grader Tracy Ryan, newly chosen drill team secretary who was transferred from Oakton High School. "I didn't want to leave all my friends at Oakton," Ryan said, but looking on the bright side she added, "It was a 25-minute bus ride and I can walk to school here."

The current school reshuffling has "split Reston in half" said Mark Melnick, trombonist in South Lakes' new band who was at Herndon High last year. Melnick said his friends from the Lake Anne section of Reston will continue to attend Herndon, at least for the next two years.

While many of the students at South Lakes may be confused at first, reluctantly transferred into a partially finished school where they will be in new classes, on new teams and forced to form new friendships, the teachers and staff at South Lakes are all there by choice.

For the 80 staff positions at South Lakes - excluding the Sunrise Valley school on the ground floor - there were more than 700 applicants. Broaddus, who interviewed most of them personally, said it was "kind of an overwhelming" rush at the school which he could not explain.

A number of teachers said they had applied because it was a fresh new school, they knew and liked Broaddus during his 12 years as principal at Marshall High School and because it was close to Reston where many of the new staff live.

"I'm only two minutes away now," said Linda Caldwell, one of three librarians who had been working nonstop for the preceding two weeks and on Friday was strenuously sorting out boxes and bookshelves.

There are 800 cartons of new books soon to arrive, she said, but the only thing that arrived on Friday was ice cream, and that had to be returned because the school's refrigerators weren't working.