You can tell them by their neatly-typed reports, complicated charts, computations and projections. By day, they are energy experts, systems analysts, efficiency experts or administrators who work in the far reaches of the federal bureucracy. By night, they are parents of school-age children offering their "consulting services" free of charge - and usually unsolicited - to the Arlington County school board.

They are especially vocal and persuasive when the board considers a controversial matter, such as closing two of the county's six junior high schools. In the past three weeks, no less than five elaborate reports on the proposed closings have been presented to the board by these professional parents.

John S. Bush, for example, works for the U.S. Gypsum Co. and has two children at Taylor Elementray School. He recently contributed a report on projected future energy costs for Arlington schools to show the board which schools are the least energy-efficient.

He said he took his data from energy use and cost records of the school board and projections from energy Users Forum Steering Committee, of which he is a member. The committee, Bush said, coordinates energy policy decisions of approximately 800 of the largest U.S. industrial energy consumers.

His paper compared energy efficiency of the county's junior high schools and predicted a 600 per cent increase in electricity costs for the schools by 1979.

The report also provoked a response from school staff that outlined utility costs per square foot in each of the junior highs and prompted a suggestion from board member Ann Broder that the board form an energy task force to study the impact of rising energy costs on the school system.

David R. Seidman has one child at Glebe Elementary School and just completed a three-year assignment at the National Science Foundation, which included helping develop a manual to assist school planners in making enrollment projections.

As might be expected, Seidman disputed the enrollment projections the board had been using to determine pupil populations school by school, saying no sufficient error analysis was conducted.

"A somewhat simplified use of a standard error propogation formula indicates that the average percentage error for a seven year projection will be seven times that for a one-year projection," Seidman told the board. "Therefore, the estimated error for 1083-84 projections is 38.8 per cent."

The board has since rerun their enrollment data to obtain new projections.

But Dr. Nozer Singpurwalla, a Taylor parent, takes exception to the board's new projections. Singpurwalla is a professor of operations research at George Washington University whose background is in systems planning and statistical analysis.

Singpurwalla says the enrollment projection computer model has no defind "confidence limits" (a margin of error to be taken into account when making projections), that the methods of projection enrollments - the cohort survival method - assumes an unchanging situation, and that the model allows for "interventions" or unexpected occurrences such as the influx of Vietnamese pupils two years ago.

The school board invites Singpurwalla to meet with its committee on pupil enrollment, which already includes several Arlington residents who are specialists in statistical projections. Among the members are an assistant secretary of defense, and professionals from the Office of Budget and Management, NASA, Bureau of Defense and George Washington University.

Seidman, Singpurwalla and Bush are but a few of the professionals who work in Washington but live in Arlington and turn up at board meetings to combine parental concerns with professional expertise.

"I would call Arlington an extremely unusual school district in that we have a lot of experts in a lot of different areas," said board chairman Diane Henderson. "It's been like this for years. These parents challenge the school board but also give it a great deal of help. It's good for the schools and good for the country that we have those experts who will put in their time and effort to school affairs."

Henderson herself joined the school board after serving on the board's advisory council for technology in the schools. Henderson designs computer systems for the Mitre Corp. Part of her job is to research potential uses for computers, and her experience proved helpful in adapting computers for education.

Ann Broder said expertise among Arlington parents "works both ways."

"It's not as though we haven't had Arlington parents who are professionals working with the school system before," she said. "The enrollment projection model we developed three years ago was done by similar types of parents. But the school population here changes rapidly, and now we have whole new sets of analysts out there who dispute what's gone before. If we can harness that expertise it can work very positively for the schools. It's a vast reservoir we can tap."

"The level of thoroughness and sophistication we get in parent committees and reports is something that would cost millions of dollars for a consulting firm to do," Henderson added. "We're very fortunate as a school system to have the kind of parents we do."