The knots of party-eager teen-agers start collecting in the convenience store parking lots at dusk on Friday evenings. Some walk boldly into the store, slap their money on the counter and stroll out with a six-pack of beer. Some come armed with fake identification cards, their tickets to illegal beer purchases. Others hang out, waiting for a willing older accomplice who can buy liquor legally and then make the drop to underage kids outside the store.

"It doesn't take a super-secret spy network to see what's going on," said Capt. Andrew P. Page of the Fairfax County Police Department.

For thousands of Northern Virginia teen-agers, alcohol is an entrenched part of their lifestyles. Alcohol has overtaken marijuana and other substances as the drug most abused by teen-agers, say school and law enforcement officials.

"Most teen-agers are using alcohol to some degree," said Mel J. Riddile, coordinator of substance abuse prevention for the Fairfax County School System. "The kids that choose to be alcohol-free are in a small minority."

About 93 percent of all youngsters 12 to 17 years of age report that they use alcohol to some degree, local and national surveys show.

"In fact, 41 percent of all high school students report having drunk five or more drinks at one time in the two weeks before a survey , making them legally intoxicated," Riddile said.

Riddile and others who monitor the problem blame the rampant use of alcohol by teen-agers on its easy availability and social acceptance.

"Show me a kid who hasn't got access to beer on a weekend and I'll show you a loner," said Page.

Parents and school officials frequently criticize police for being too lenient on underage youngsters who purchase alcohol, and on the store owners and clerks who sell to the teen-agers.

Capt. Page acknowledged that the police frequently turn their heads.

"If we went through the legal process with every adolescent we saw with beer, we would have all of our officers tied up in court dealing with juveniles," said Page. "It doesn't seem reasonable to take officers off the street to chase 16-year-olds with a six-pack when the community needs them for more important things."

Last year, Fairfax County police arrested 444 juveniles for possession of alcohol or drunkenness in public. Four adults were charged with selling or giving liquor to minors.

The Virginia Alcoholic Beverage Control Commission also monitors alcohol sales and purchase laws. Last year its investigation and enforcement agents arrested 883 juveniles state-wide for illegal purchase or possession of alcohol, records show. The ABC also charged 105 store owners with violations of liquor sales laws.

Page termed the arrests "a spit in the ocean" in relation to the number of illegal sales and purchases of alcohol each year.

Law enforcment and school officials accuse store owners of doing a sloppy job of policing liquor sales.

"I don't think the stores are doing much to check teen-agers," said Robert Hanley, principal of T. C. Williams High School in Alexandria. Hanley said he gets frequent calls from store owners asking him to stop teen-agers from buying beer in their establishment.

When Hanley replies that the responsibility lies with the stores where the purchases are made, he said, "they tell me the clerk is nervous about asking kids for IDs."

But even when store clerks demand identification cards, many teen-agers produce fakes. Some youngsters say they borrow IDs from older look-alike friends. Others say they change the birthdates on their own IDs and relaminate the cards. And some teen-agers say they obtain false IDs through quickie passport-photo shops.

School officials say the drastic increase in the use of alcohol by students coincided with the lowering of the legal drinking age in Virginia, which went from 21 to 18 for beer and wine in 1974. In 1981, the legal age went up to 19.

"When it was 21, an 18-year-old could pass for 21," said police officer Page. "If it was 18, a 16-year-old could pass for 18. And having it at age 19 is like trying to cure cancer with a bandaid."

But one of the major sources of alcohol for teen-agers is their parents.

"Parents say there's no way they can have a party for their high school children without having beer these days," said Carmin Caputo, a member of the Fairfax County School Board.

"The kids say if they gave a party without alcohol, nobody would show up," said Riddile.

Principal Hanley criticizes parents for giving in to their youngsters: "They serve alcohol because they're afraid if they don't their kids will go off somewhere else and do it. Some parents are easily intimidated by their kids."

Other parents don't consider alcohol use much of a problem.

When Hanley called one mother after he caught her son drunk at school during lunch, he said he got a long sigh and an "Oh, thank God he's not smoking dope."

"They don't think it's serious because they drink themselves," said Hanley.

A growing number of parents are becoming concerned with the problems of alcohol, however. But many are finding there are few places to turn for help.

School officials admit they are doing an inadequate job of addressing the problem.

"We're finding kids who need treatment at 12 years of age, and we have nothing for them," said Riddile. "We're having to deal with a problem we're not prepared to deal with."

The number of students suspended for drinking at school has tripled in Fairfax County in the last five years, from 100 suspensions to 329, school records show. In Arlington, student suspensions for drinking on school grounds doubled during the same period, from 15 to 31.

Riddile said those figures reflect only a fraction of the problem. "We don't know what's under the surface," he said.

The Fairfax County School System, with the 11th largest enrollment in the nation, has places for only 10 students in its program for youngsters with severe alcohol problems. And there is only a handful of private and community programs available to help children who abuse alcohol.

Last year almost 60 parents in Fairfax County sent their children to a special program in Florida for alcoholic teen-agers.

"The fact that we're sending six times the number of kids to Florida that we are treating in our own county tells me we need something," said Riddile.

Some parents have formed their own self-help organizations. Toughlove, a group that tries to help parents cope with troublesome children, including alcoholics, has ballooned in less than a year from one branch of five parents to 12 units with 300 parents throughout Northern Virginia.

Parents involved in Fairfax County school groups have organized 26 units of Parents Who Care, a group trying to battle drug and alcohol abuse among students.

And area school systems are slowly integrating alcohol abuse prevention programs into the curriculum. This year Fairfax schools introduced drug and alcohol education programs in grades 1 through 6 at four pilot schools.

"Some of the elementary children already are very street-wise," said Marie Sterne, elementary health and physical education program specialist. "Some have been exposed to drugs and alcohol at home or in the community. We want to get them involved in prevention education before the first joint is passed around."

The program, approved by the School Board in 1979 for use this year, begins in the first grade with discussions on household medicines, graduates to tobacco in the fourth grade, alcohol in the fifth and drugs in the sixth. Throughout the program, teachers attempt to coach the youngsters in dealing with the peer pressures that often lead to abuse, said Sterne.

The curriculum is scheduled to be introduced in all 120 of the county's elementary schools next year, she said, and expanded to kindergarten programs the following year.

School systems in Arlington, Alexandria and Fairfax have begun appointing administrators and counselors to deal with the mounting alcohol problems of their students.

The alcohol problem of students in Northern Virginia parallels that of school systems throughout the country, school officials say.

"If you live anywhere in the United States, you have a problem," said Riddile. "There is a whole generation at stake, and we better do something about it."