Months in advance of the Fairfax County high school graduation season, police began developing plans to handle problem parties, a strategy that included using young-looking plainclothes officers to infiltrate those events without warrants.

That strategy and the guidelines the police developed are perfectly legal, according to several scholars, but have nonetheless generated passionate arguments since several underage drinkers and the hostess of one party in Vienna were arrested.

Because the Oakton High School party at the home of Robert and Retha Morgan appeared to meet legal criteria -- with uniformed police reporting that merrymaking had spilled into the neighborhood and lack of control over the flow of guests -- the law enforcement officers were within their rights, said Fairfax Commonwealth's Attorney Robert F. Horan Jr. "Historically, the {U.S.} Supreme Court has always taken the position that what is readily apparent, in plain view, is fair game," Horan said.

He added that it was unrealistic of the Morgans to expect that the party would remain private -- at least without better chaperoning. "When you invite the senior class," he said, "you are ostensibly inviting their girlfriends, boyfriends, near and far friends, and everybody who wishes they were a senior at Oakton."

Several law school professors and lawyers knowledgeable about constitutional law have said that, in their view, the legality of the police action hinged on the partygoers' expectations of privacy and the officers' reasons for believing that illegal acts were taking place.

"The question in this case is, whether the party that was being held . . . was so broad as to include anyone who walked in off the street," said John Kenneth Zwerling, an Alexandria lawyer and a board member of the National Association of Criminal Defense Attorneys.

But Zwerling, who emphasized that his knowledge of the incident is secondhand, said that even if the police action did not violate the constitutional prohibition on unreasonable searches and seizures, some people will ask: Was it appropriate?

Wayne LaFave, a University of Illinois law professor and a nationally recognized expert on search and seizure, said the courts are not in agreement when it comes to police operations.

But he said the Fairfax case, as described to him, sounded within the bounds of the law. "I don't think there's a single court in the country that would find that kind of police activity improper, at least in the Fourth Amendment sense," he said.

"It gets a little tricky with a private home," he added. "But I'd say that if you have a party with mobs of people flowing in and out . . . you can't have had much of an expectation of privacy if you weren't really checking."

Horan also said the plainclothes operation did not violate Virginia's trespass law. According to state law, a person is permitted on private property unless specifically asked to leave, he said. "We're not talking about walking into somebody's bedroom or bathroom," Horan said. "We're talking about walking onto their property."

Not everyone agrees. "I think it's outrageous that the police can come into anybody's home they want to when there's a party going on unless the people have a bouncer at the door," said Alexandria lawyer Marvin Miller. If the police were that concerned about drunk driving, Miller said, "they should have stopped all partygoers at a roadblock instead."

"There are always serious problems with violations of the law, but it doesn't give the police carte blanche to thumb their nose at the Constitution and ignore the restraints which exist," Miller said. "The end does not justify the means in a free, democratic society."

Fairfax police infiltration of teen-age parties dates to the early 1970s when masses of students would gather in fields and drink beer. At that time, police sought advice from the commonwealth's attorney's office, and were told that under certain circumstances, plainclothes operations were legal, Horan said.

Police officials in Montgomery County and the District have said they do not use plainclothes officers for teen-agers' parties. In Virginia Beach, however, such officers are used on occasion but large teen-age parties are not as prevalent as in Fairfax.

These parties tend to start small, police say, but some can mushroom out of control, occasionally creating a variety of neighborhood problems.

"We get many, many complaints of trashing, urinating in people's yards, shrubs torn up, vehicles tampered with," said Fairfax Police Chief John E. Granfield. He and others said the most serious threat takes place when the party ends, and hundreds of teen-agers, some of whom have been drinking heavily, head home.

Police Capt. Richard J. Rappoport, commander of the Reston District station, said that under the guidelines police drafted in advance of the graduation parties, uniformed officers on patrol were told to alert two teams of plainclothes officers if they saw potential problems at open-type parties, such as suspected underage drinking, so they could swing by for a closer look.

Rappoport said that last week the teams of three visited 10 of the large teen-age parties on Thursday, Friday and Saturday nights. But he stressed that officers did not infiltrate all of the parties because they did not fall within their legal guidelines, and at some of the others they infiltrated they did not observe violations and promptly left.

Before the Morgans' party, police consulted with the commonwealth's attorney's office to clarify what their rights were and which parties they could visit legally. Rappoport and others said the main reason for using plainclothes officers was to identify parents who are serving liquor to minors or allowing minors to drink on their property. As a result, the Morgans were arrested by the Virginia Alcoholic Beverage Control Board agent and charged with aiding and abetting the consumption of alcohol by minors.

In Virginia, the legal drinking age is 21, but youths born before July 1, 1966, were grandfathered in when the age was raised.

Although Fairfax police made a few other arrests last weekend, the events surrounding the Morgans' pool party for the 619 graduating seniors at Oakton High became the subject of public scrutiny because the Morgans complained about the police action.

Retha Morgan has said that she had done everything possible to keep her party orderly, including informing police in advance and receiving a booklet of party tips from them. Morgan said she posted "no alcohol" signs in hopes of discouraging drinking.

Morgan maintains the party was orderly, and that if teen-agers were drinking, they were doing so out of plain view.

Police counter that the uniformed officers who were patrolling the neighborhood saw several indicators that the party was out of control and minors were drinking.

"Kids were clearly walking in and out with beer," said Rappoport, who was briefed about the party by his officers. "There were kids staggering."

He said there were "beer cans and bottles everywhere," and initially at least 50 teen-agers were milling around neighborhood streets, some of whom appeared to be drunk. Neighbors interviewed after the incident disagree on whether the party was out of control. Rappoport said the department did not receive any complaints during the party.

After the plainclothes officers were sent in, they reported seeing underage teen-agers drinking in front of the Morgans. They also said they heard someone announce on the disc jockey's public address system that guests could drink either the nonalcoholic beverages available or whatever they had brought.

Rappoport said the two Fairfax plainclothes officers and one plainclothes ABC agent who attended the Morgan party did not see "no alcohol" signs posted. The ABC agent arrested several juveniles for alcohol-related violations on the Morgans' property.

Although county police did not make arrests at the party, they arrested seven juveniles at a roadblock a quarter-mile from the Morgan house in the 10000 block of Old Hunt Road for either alcohol- or traffic-related offenses, Rappoport said. He said police did not want to "rain on their parade," but they also did not want teen-agers driving home drunk. He said the roadblock was set up so that officers could dump out alcohol found in the vehicles and ensure that the drivers were sober.

Retha Morgan said that at 3:30 a.m. she walked to the roadblock to find out what was going on and to see if anyone needed a ride home. Morgan, who was carrying an empty glass, flunked several routine field sobriety tests, such as touching her nose, said Rappoport.

Before charging her with being drunk in public, police checked her blood-alcohol content to ensure that she was failing the field tests because she was drunk and not because she was tired, Rappoport said. The blood-alcohol content test is not typically administered for the drunk in public charge, police said.

Rappoport said Morgan registered 0.16 on the test -- above the presumed intoxication level of 0.10 in Virginia -- and was ordered to spend the night in jail.

Richard Titus, 44, an Oakton parent whose teen-ager attended the Morgan party, commended the Fairfax police. "There is no privacy issue, the beer was being lugged from blocks away," he said. Titus alleged that the Morgans' behavior was "totally irresponsible, totally contrary to what the schools have been trying to do."

Peter Kilitka, a member of a nonpartisan citizens police advisory group, also applauded the police. "I think what they did was warranted, and they deserve credit . . . anyone who invites 619 guests to a private residence has to be irresponsible. True, 619 didn't attend. But, even 200 is irresponsible."

According to Richard H. Schwartz, a clinical professor of pediatrics at Georgetown University, with a subspecialty in drug and alcohol abuse, 85 to 90 percent of teen-age partygoers will drink if alcohol is present.

Patricia R. Smith, president of the 350-member Parents Association to Neutralize Drug and Alcohol Abuse, based in Nothern Virginia, said her group supports what the police did.

Smith said parents cannot expect to invite that many teen-agers and keep control unless organized activities are planned, such as games. "You can't just put them in a back yard and expect everything to be hunky-dory."