Don, a junior at Chantilly High School in Fairfax County, claims that six years of daily marijuana smoking have helped him become more interested in life. At home there is frequently "nothing to do," but a walk in the woods to smoke marijuana, Don says, fills his mind with "80 million things to do that I never thought of."

Cynthia, a senior at Fort Hunt High in Fairfax, says she has smoked marijuana four times a week for four years and argues that "pot won't mess you up unless you are going to mess up anyway."

These views - sharply challenged by a number of medical authorities - may help account for what an authoritative University of Michigan survey calls a sharp increase in daily marijuana use among high school seniors since 1975. According to the study, widely regarded as the most comprehensive and accurate assessment of American teen-age drug use, one of 11 seniors now smokes marijuana daily.

The findings are alarming to many medical and drug authorities, who express fears that frequent marijuana use may have many, harmful long-term effects on the students. Many high school students dismiss these fears and refuse, for instance, to consider marijuana a drug.

"Marijuana," insisted one Fairfax County student, "isn't a drug. It's just something that's around, sorta like blue jeans."

The percentage of the nation's high school seniors who smoke marijuana daily has increased from 6 percent to 9.1 percent, according to the University of Michigan study.

"Researchers don't know about the long-range ramifications of these figures," said Dr. Robert C. Peterson, assistant director of the research division of the National Institute on Drug Abuse in Rockville. "I think, though, there is reason to be worried. Show me any example in history where a drug made people any better able to cope with the difficulties of living."

The study of high school seniors shows that marijuana is almost universally available to high school students, that students are starting to smoke marijuana at increasingly younger ages, that more than half of them have smoked it, and that nearly two-thirds think there is nothing wrong with smoking it.

"Smoking pot has gone from being a deviant behavior to a majority behavior," said Dr. Lloyd D. Johnston of the University of Michigan who directed the drug-use study.

While marijuana usage has increased sharply in high schools, use of almost all other drugs has remained the same as in 1975 or declined, the study shows. Troubled by drug problems in the suburban Washington school system, Fairfax County police said recently they will place undercove agents in the county's high schools.

Interviews with students at three high schools in different areas of the County, which has a highly rated school system and an annual median family income of $28,500, indicate that regular marijuana users believe the drug enriches their learning about academic subjects and about "life" and gives them a "distance" from their problems that helps them relax. Drug researchers dispute the students' views.

Regular-marijuana smokers interviewed in the county agreed that too much marijuana is dangerous. "Pot is just another obstacle you have to deal with in growing up," said Cynthia from Fort Hunt High.

Marijuana for teen-agers is what martinis and Valium are to the teenagers' parents, according to Dr. Allan Cohen, a clinical psychologist for the Pacific Institute for Research and Evaluation in California, a center for drug abuse information funded by federal grants.

Cohen, who has spent 15 years in marijuana research, said "undoubtedly some adolescents are self-medicating themselves against stress." But Cohen, along with other drug researchers interviewed, said most high school students use marijuana to fill in the empty spaces in their lives.

Students in the Fairfax high schools agreed that when they face a boring class or an empty afternoon at home after school before their parents return from their jobs nothing fills the hours like smoking marijuana.

"I figure if you have to go to some idiotic class," says Teresa, a 16-year-old junior at Fort Hunt High, "you might as well get high and enjoy it." At Chantilly High, a 17-year-old Christy says, "If I have a test and I know I'm not going to do well, instead of sitting there (in class) and fretting, I might as well enjoy it."

Cohen said high school marijuana smokers use the drug to endure boring classes, to make up for a lack of goals and, often, simply to talk with each other.

"In the old days, it was possible that kids would get together and talk and enjoy each other's company. Now that group (regular marijuana smokers) finds it hard to communicate unless they are stoned. Then they really don't have to communicate," Cohen says.

Although no long-term studies of high school marijuana users have been conducted, Cohen said that clinical evidence and information gathered by counseling agencies indicated that chronic marijuana usage is related to decreased academic performance, truancy and a decreased ability for objective self-appraisal.

"What researchers and teachers are worried about," Cohen said, "is that this historically unique amount of daily intoxication may postpone maturity. Adolescense is a time when people must learn to make decisions, resolve conflicts and form their identities."

Researchers such as Cohen, Peterson at the National Institute for Drug Abuse and Johnston at the University of Michigan say they think regular marijuana smoking by high school students may stop students from "growing up" as they grow older. "The real world does not have much patience with a 22-year-old masquerading as a 16-year-old," Cohen said.

Students interviewed in Fairfax high schools claim marijuana has none of these adverse effects, unless it is abused. They often cite newspaper and television reports that say researchers have found no dangerous short-term effects on health from moderate usage. Smoking marijuana daily, they say, is moderate usage.

"When I'm high," says a 17-year-old junior from Fort Hunt, "I'm in a good mood for learning. I never get too excited for a test or too depressed to pay attention.

"Growing up is hard. There has a lot of ups and downs. Now that pot has been introduced in the high schools, things have calmed down."

Dr. Keith Schuchard, an English teacher at DeKalb Junior College in Atlanta and the mother of a 15-year-old who used to smoke marijuana regularly, said the idea of an adolescent who does not get either excited or depressed is repulsive.

"The whole 'laid back' thing when you see it in a 14-or 15-year-old is not very pretty," said Schuchard, who organized 40 families in an affluent suburb of Atlanta to stop teen-age marijuana use.

She said that she and other neighborhood parents, whom she did not know until the marijuana problem emerged, "dismantled a drug-oriented life" for her son by eliminating some of the vacuums marijuana filled for him.

The neighborhood parents group, which teen-agers called "PSP" "Parents' 'snoop Patrol," did not allow unsupervised parties, unaccounted for spending money or "hanging-around time."

The program worked, Schuchard said, and she is now instructing parents in other Atlanta suburbs about how to start up their own PSPs.

In Fairfax, high school students say such a program would not do any good because parents do not have enough time for that kind of supervision and marijuana smoking is not a problem that parents can solve.

Students interviewed agreed that they are the ones responsible for learning how to use marijuana.

"It's like learning to drive a car," a Chantilly High junior says. "Some kids can't figure it out, but most everybody else has no problems."