Pentagon workers wear photo identification badges to gain entrance to their supersecret offices. So do employes at the CIA and the White House.

Beginning this fall, some Arlington high school students will start carrying them, too--but with one small difference. While their adult counterparts need the cards to get into restricted areas, the Arlington high schoolers will need to produce their ID cards if they want to get out.

It's all part of an effort, started by the county School Board two weeks ago, to phase out the county's so-called "open campus" policy. Critics had charged that the policy, which allowed students to leave school grounds at lunchtime with written parental permission, had contributed to an increase in juvenile crime and drug and alcohol abuse.

"I don't think ID cards are going to be as big an issue as they might have because the idea originally came from students and parents," said School Board member Margaret A. Bocek. "It might have been different if the school board had dreamed it up and imposed it. But I don't think the students should have to wear these things after we phase out."

Under the new Arlington County policy, which takes effect in September, freshmen will be prohibited from leaving their high school campuses during lunch hour while sophomores, juniors and seniors will be allowed to leave only with a photo ID card. The following fall, the ban will include freshmen and sophomores, and so on in subsequent years until no Arlington high school students will be permitted to leave campus at lunchtime.

Students who have valid reasons for leaving, such as those involved in work-study or career center programs, will not be affected by the new rules.

A permission slip signed by parents at home will no longer be adequate for students who want to leave school at lunchtime. Parents will have to go to the school to sign the slips and foot the bill for a photo ID.

"We're not going to become a police state," said board member Simone J. Pace. "The idea is to have some simple, readily visible means of identifying which students really are allowed to leave the grounds."

Part of the problem with the old policy, board members said, was that some students did not return to classes after leaving for lunch.

"We are responsible for those students during the school day," said Pace. "This will help foster good attendance habits and what you wind up doing is creating habits that carry over to college and work."

Fairfax and Loudoun counties have policies forbidding students from leaving school grounds at lunchtime, while Alexandria's T.C. Williams High School has a policy allowing students to come and go freely. Student IDs at T.C. Williams, where the student body consists solely of juniors and seniors, are used primarily for admission to school functions such as dances.

Falls Church allows seniors in their last quarter to leave school grounds for their lunch period if they show a valid school ID. Prince William County does not allow students to leave the campus for lunch.

The Arlington School Board saw vigorous opposition to eliminating its "open campus" policy, most of it from students and parents who contended that the vast majority of students, who obey the rules, should not be penalized for the abuses of the minority.

Law enforcement officials and many parents favored the change, saying that rude and unruly students have disrupted neighborhoods and littered public areas near schools.

"A lot of students are not participating in that unruly behavior ," said School Board Chairman Evelyn Reid Syphax. "Unfortunately, life is like that and others sometimes have to be penalized when only a few are abusing the privileges."

Syphax said the new policy will "probably mean several more staff people will have to be hired so we can establish a better monitoring system."

Still, some board members voiced concern over whether the new policy would actually serve to deter student rowdiness at noontime.

"If the students are allowed to go out of the building, but must stay on campus, how do we keep them on campus?" asked board member Michael E. Brunner. "Obviously, we're not going to have guards stationed every 20 feet.

"The question is, 'How do we enforce it, and can we enforce it?'" he said. "And if you can't enforce it, then what have you done?"