There are cigarette butts on the floor, ashes on the sink and hints of tobacco smoke in the air in the girls' restroom at Reston's South Lakes High School. Teacher's aide Kay Fletcher, who is paid about $9,000 a year to notice such things, frowns and sniffs. "They've been here," she says.

"Here, too," she sighs, walking out of another lavatory moments later. "They've dumped the ashes all over the place."

From the moment South Lakes High opened its doors in 1978, tobacco was banned. Under Fairfax County school board edict, students and faculty alike were forbidden to puff, dip or chew. South Lakes was the board's grand experiment at eliminating the golden weed. Since then, six other county high schools have followed suit, and Fairfax now spends $133,000 a year on a full-time, 14-member cigarette squad -- unique in the Washington area -- to enforce its ban.

Administrative optimism aside, the fact is that at South Lakes High, smoke still gets in your eyes. Smoking may be banned, but it has not been snuffed out.

"We have a system," says smoker Monica Hall, a South Lakes senior who estimates there are at least 60 students who regularly light up in various alcoves, lavatories and drainage ditches. "You go to the last stall in the bathroom, and flush it down if you hear someone coming."

"You can stop people from smoking in a certain area," says Fairfax Supervisor Marie Travesky, who was an early critic of the program, "but if people have a smoking habit, they're going to find a place to smoke."

Even so, most Fairfax school officials say the no-smoking program is working. They never expected to eliminate all smoking, anyway, and they claim vandalism and drug use have declined significantly at the schools. The ban also shields nonsmoking students from considerable peer pressure. "There are always going to be people who sneak off for a smoke, but at least it's the exception and not the rule," says Supervisor Audrey Moore, a champion of the antismoking effort.

Those reports no doubt influenced the Montgomery County School Board's decision last week to ban smoking in all high schools in the county, an action that was criticized by teachers, parents and students who argued that the prohibition would prove costly and virtually impossible to enforce. Some board members predict the action will be overturned when Montgomery's new board convenes in January.

Alexandria, Arlington, and Prince George's still allow student smoking in specially designated areas. The District and Montgomery now prohibit it altogether.

And around the Capital Beltway, there is little consensus on whether to ban or not to ban. "If you get rid of smoking areas, kids smoke in all the wrong places," says principal Joseph Hairston of Crossland High School in Prince George's County. "If a kid wants to smoke, he's going to smoke."

"Eliminate smoking areas and you eliminate a convenient gathering place for marijuana use as well," counters Fairfax School Board Chairman Ann Kahn.

In Garden City, Kan., smoking or possession of cigarettes means an automatic five-day suspension from high school, "Yes, they still smoke," says Garden City assistant superintendant Ronald Lantaff. "There's no way to actually stop it."

The nationwide percentage of high-school-age students who smoke at least a cigarette a day has dropped from 29 to 20 percent in the past five years, according to the Public Health Service. But a study by the National Institute on Drug Abuse last year found that about 13 percent of high school seniors still smoke a half a pack a day or more.

That's exactly why Fairfax imposed the ban at his school, says South Lakes principal George W. Felton. "Any child who can't go a school day without a cigarette needs our help."

Fairfax has never attempted a county-wide ban, instead experimenting with South Lakes and extending the program only where principals and the community have asked. School officials attribute a good measure of their success to the money the county appropriates each year for enforcement.

South Lakes is a nicotine addict's nightmare. Not even the faculty is permitted to smoke on school grounds. Only six out of 100 faculty members admit to the habit, according to school officials. No employment interview is complete without the smoking question. "We tell them that if they smoke, they may have a problem here," says senior principal Stephen Wareham the administrator of the no-smoking enforcement program.

The county's "awareness aides," or "smoking narcs," as students call them, don't search pockets or confiscate, but they do enforce an elaborate code of sanctions. In addition to suspensions, first-time offenders at South Lakes must attend several antismoking "awareness" classes. There are equally severe sanctions for students found chewing tobacco or dipping snuff, a new, but growing, phenomenon among high school boys.

Felton is something of an evangelist on the subject. "I've never smoked, smoking annoys me. I don't want to die of lung cancer. Kids get caught here and they expect a big moral number from me. I just tell them: 'It's your choice. You want to die early, there'll be more room for the rest of us. But you're not going to do it in my school.' We can't cut it out 100 percent here, I realize, but we can make it pretty tough."

At South Lakes, smoking infractions have dropped from 164 in 1980 to 112 last year, according to Wareham. Whether that means students have stopped smoking at school or just gotten smarter, Wareham can't say.

Students say that while some may have stopped smoking, most are just more careful. And while officials have no qualms about dictating the health habits of the teen-agers in their charge, many students in Reston and elsewhere are ambivalent.

"It's all right," says a freshman boy from Reston who was spied dipping snuff near the South Lakes drainage ditch. "I didn't find out about it until my mother got a letter from the principal, and then I got it. I still dip, but not at school."