For Arlington school board member Margaret A. Bocek and her husband, the first day of school this year began late Monday night when they and 40 other parents camped out on the lawn of the county's Page Traditional School to ensure that their 3-year-old children could attend there on opening day, 1984.

"I think it's group psychology," said Page principal Frank Miller. "People saw a line forming and thought they needed to get in line to get in."

Whatever the explanation, there were only 25 openings for the 1984 Page kindergarten class, the entry level that would ensure continued enrollment through the seventh grade at the back-to-basics school, and these parents took no chances that their childrens' names would not be at the top of the list.

Late-comers were put on the waiting list, Miller said yesterday afternoon, shortly after Mary Jo Mitchell left the school with 3-year-old Jeffrey in tow. "We're Number 75--don't you love it?" Mitchell sighed.

In the last three years, such parent stakeouts have become commonplace at Page, a public alternative school that stresses a traditional format of self-contained classrooms, regular homework and strict standards for behavior and appearance. Page parents have been lobbying recently for expanding the program to the eighth grade and for expansion of the school's program to other schools.

Admission to the 370-pupil school in the past was by lottery or by giving preference to the brothers and sisters of students already attending Page, but today it's on a first-come, first-served basis.

Bocek and her husband, Robert, were lucky. They got Number 16, after a camp out that began at 9:30 p.m. Monday and ended at 7:15 a.m. yesterday. Margaret Bocek kept their place in line once while her husband went home for a blanket and lounge chair. When he returned at 11 p.m., she went home to relieve the babysitter. She returned at 6 a.m. with coffee, freeing her husband to go to work.

"We all agreed it was outrageous that parents have to stay out overnight and have such a tremendous waiting list to get in to the school. Most of the parents wanted to know why," said Bocek, who joined the school board in July.

The Boceks' daughter, Elizabeth, 5, started at Page yesterday and they wanted to make sure their son, Bobby, 3, would be able to attend the same school two years from now, she said. "I felt bad, being a board member and being with other parents, that I could not give them assurances or an answer that is responsive about the future."

The school board is about to begin deliberations on possible school closings for the fall of 1984 because of declining enrollments. As part of that process, the board will also review the role of the county's three popular alternative schools -- Page, with its back-to-basics, and Drew Elementary and H-B Woodlawn Secondary, with a less structured learning environment.

While there haven't been camp outs at H-B Woodlawn or Drew, the principals at both schools said yesterday that they, too, have waiting lists. Woodlawn's 71 slots for seventh graders entering in 1983 are filled and applications are coming in for 1984. Drew already has a waiting list for first graders wanting to enter in 1983 and 1984.