As is so often the case, it began as an occasional, apparently innocuous activity, a joint of marijuana smoked in the company of friends on weekends or after school. Gradually, it became a habit, then virtually an avocation.

Midway through secondary school, the attractive and intelligent student at Marshall High School in Fairfax County was sneaking a joint between classes in the girls' rest room, in the parking lot or elsewhere on the school grounds during lunch break.

By the end of the school day she routinely was so stoned she couldn't concentrate on anything, much less her studies. High school had, in fact, become a perpetual high, obscured in a cloud of marijuana smoke.

The girl is, of course, atypical of high school students in Fairfax County, or in Northern Virginia for that matter. Estimates of the number of students whose drug use is heavy enough to impair seriously their school work run from 5 to 10 percent, but the incidence of occasional use is much higher.

"You find it at almost every school function," said one Fairfax teacher, "extracurricular activities, dances, sports events. You can smell it all the time in the rest rooms at school. It's just so accepted among the kids. If you say so-and-so was high today, they just laugh and say, 'What else is new?' It's much more prevalent than most people think."

Last March, declaring that illegal drugs, chiefly marijuana, had become more readily available in schools than on the streets, police and school officials in Fairfax announced an undercover campaign against school drug use. Posing as students, agents would ride the school buses, attend classes, eat with students in the lunch rooms and ferret out the drug traffic.

Now, nine months later, police and school officials are still committed to the use of undercover agents, although they are closemouthed about exactly where they have been used and how often.

"It's between the principals and the police," said Fairfax Superintendent S. John Davis. "An agent stays at a school until he's been identified or made his arrests. Then he moves on."

Under confidential guidelines approved by the school board, agents were allowed to pose as students or uniformed service employes, but not as teachers, according to school board records. Arrests, when warranted, were to be cleared with the principal and, whenever possible, made at home after school hours, not on school property in front of large numbers of students, the records show.

Fairfax Police Chief Richard A. King says his department does not keep separate statistics on the number of school-related drug arrests, but he says arrests are being made and he believes the program to be effective.

Nearby Alexandria also uses undercover police officers posing as students to crack down on drugs in school, although, as one school official said, "we don't like to publicize it very much." During the last academic year, 59 students were suspended from Alexandria schools for drug use and a police record was made in each case.

Arlington assigns a uniformed officer on a permanent basis to its high schools to investigate drug use, among other things. Arlington schools suspended 49 students last year on drug-related offenses, most of them for use or possession of marijuana.

In Fairfax, the plan to use undercover agents originated about a year ago, said Davis, after it became increasingly apparent that drug use in the 22 county high schools had become widespread and, at least for some students, was seriously interfering with the educational process.

"We decided we had to take some action. We decided maybe we had been lax," said Davis.

The school board secretly approved the use of undercover agents in January, but no public announcement was made until March and then officials would say only that "one or two" agents had been placed in unspecified schools.

By spring, Davis had decided to get the drug problem out front. He ordered that the May-June issue of the devoted system bi-monthly bulletin be devoted exclusively to the problem of drugs in schools.

"We see every day the tragic waste caused by substance abuse," Davis said in an article he wrote for the bulletin. "We mourn especially for the young - how much they are missing and how much of their talent society is losing when they bury themselves almost literally in patterns of abuse."

In the same issue, Police Chief King observed in an articles, "When these drug abusing youths graduate, they become society's problems. For instead of using their high school years to cope with life's problems and stresses, they have used these years meant for emotional and intellectual growth to escape from reality. Their ability to take a responsible adult role in society is being seriously impaired by drug abuse."

Robert B. Davis, principal of Chantilly Intermediate School, commented, "We accepted as fact that drug use affected a sizable number of our students and was not confined to the unmotivated or troublesome ones."

Both King and Superintendent Davis say the main target of the undercover campaign has been drug trafficking on school property. They both say they believe their effort has achieved at least a modicum of success.

But many teachers and students say drugs are still available in schools and, on the surface at least, little has changed from a year ago.

"It's pretty much the same now as it was last year," says Mark Gorenflo, a senior at Oakton High School and student representative on the school board. "At least the community is now aware that there is a problem, and I would like to think that parents are becoming more interested. The students who are smoking pot at school are wasting taxpayer time and taxpayer money because they're taking up a disproportionate amount of the administrators' time. Our school administrators are becoming highly paid babysitters."

"There was a lull last spring after the publicity about the undercover agents," said one teacher. "But as soon as school opened in the fall, it started up all over again. This year, it's like it's always been."

"When you get 35 kids in a class, most teachers will be concerned with the 30 who are there to get an education, not the ones that are stoned," said Don Bradford, a teacher at South Lakes High School in Reston and for five years a teacher at Langley High School in McLean.

"I've had 'em stoned in my classes. They don't think you know it. They think they're fooling the teachers. They're not disruptive as far as classroom behavior is concerned. They come in all red-eyed and listless and they don't do anything. They just kind of fall asleep. They float in and when the class is over they float out again," he said.

"There's not a whole lot the school system can do. You throw them out and three days later they're back in school again. A lot of them don't care if they get caught and their parents don't either. If they weren't into drugs, they'd probably be into something else," Bradford added.

In Northern Virginia, as elsewhere, the drug scene has shifted from LSD of the 1960s to heroin of the early 1970s to the current marijuana epidemic and, more recently, to the hallucinogen PCP, said Shaun McBride, assistant director of Crossroads, a Fairfax drug abstinence program.

"It is very widespread among adolescents," said McBride. "We see kids in here as young as 11 and 12 years old. The ones who are into PCP see it as the next step up from marijuana. They sprinkle it on parsley and smoke it."

Crossroads, which handles a caseload of 80 persons at any one time, insists on total abstinence by all participants in its programs and offers extensive counseling and group therapy. About half its clients are persons of school age or who are enrolled in school.

"Something that seems to be common to all drug users, no matter what age," said McBride, "is a low sense of self esteem. We say here that in order to start working on your drug problem and in order to start working on the rest of your life, the first thing you've got to do is stop taking drugs."