It is lunch period at Richard Montgomery High School in Rockville, and Karen Broad slips outside an unobtrusive back door next to the cafeteria to savor what is rapidly becoming a vanishing custom: visiting the high school smoking area.

"My friends come out here," Karen says, taking nonchalant little puffs on the cigarette dangling from her right hand.

Although it is sunny and not too cold, there is just a handful of students clustered outside. Karen, a 15-year-old junior, is the only one actually smoking.

But soon, even she may be unable to.

Montgomery County is on the verge of joining six other school systems in the Washington area in banning student smoking. Citing "the clear danger it represents to the health and longevity of young people," School Superintendent Harry Pitt plans to recommend today to the county's Board of Education that students be forbidden to use any form of tobacco on school grounds.

After years of preaching in health classes the evils of smoking, it is hardly surprising that school officials now are trying to make students practice tobacco-free habits on school property.

What is perhaps more surprising is that smoking in school -- once a powerful symbol of student rights -- appears to be out of favor today with many high school students.

"It is not what I would term one of those inalienable rights," said Doyle Hodges, 17, a Richard Montgomery senior who is on the indoor and outdoor track teams.

Smoking by students has declined significantly, according to national studies: A survey of high school seniors, conducted annually for the National Institute on Drug Abuse, found that the percentage who smoke at least a half a pack a day dropped from 19 percent in 1976 to 11 percent in 1986.

If the board accepts Pitt's advice, Montgomery will join the school systems in Alexandria, the District, and in Anne Arundel, Fairfax, Howard and Prince George's counties, all of which have, in the last several years, reversed the more liberal smoking policies adopted in the late 1960s and early 1970s. Prince William County schools also will ban smoking, starting next year.

"I think it should be banned," agreed Jennifer Wilder, 17, a Richard Montgomery senior who is an editor of The Tide, the school's student newspaper. "It seems in school it is not really appropriate. It is dangerous for people who don't smoke. It is also unpleasant."

Even Karen Broad, who said she smokes about three cigarettes a day, said she did not think a ban would pose much of a hardship.

Because Richard Montgomery students may leave school grounds for lunch, she said, "a lot of people would go to the 7-Eleven" nearby. But she and other students predicted a ban would have a side effect school officials undoubtedly do not intend: More students would skip classes to smoke away from school. Several principals also have said they fear the ban will drive students into school bathrooms to smoke, creating problems enforcing the ban.

Andrew Herscowitz, a senior at Winston Churchill High School who is the county's student member of the school board, said he was surprised at the reaction when the topic of smoking arose last year at a meeting of local student government leaders.

"I expected students unanimously to say there should be smoking areas. I always assumed students would think, 'They shouldn't take away my rights,' " he said. But about half the representatives supported a smoking ban, he said.

Personally, he said, he did not favor a ban -- but not simply because he believed students deserved to smoke. "Smoking is an addiction in some cases. They can't quit," he said.

The school system's current rules allow each school to set its own policy, and only three high schools ban smoking, including Seneca Valley High School in Germantown, where student government leaders suggested the ban last year when a popular football coach, Hugh McCabe, was dying of lung cancer. "This guy, this whole thing, was really on their mind," said Richard Dumais, the school principal.