When Fairfax County police sent plainclothes officers to a high school graduation party at a home near Vienna two weeks ago, they touched off a debate about the lengths to which authorities should go in responding to public pressure to reduce alcohol-related highway deaths among teen-agers.

The Police Department's action appears to have been within the bounds of the Constitution and state and federal statutes, according to legal scholars. And there seems to be no argument that police have a role in enforcing Virginia's minimum drinking age of 21 and in keeping increasingly crowded roads safe.

But certain questions -- among them, whether the use of undercover officers in this area is prudent or intrusive, whether such action is demanded by the public or oversteps the bounds of good law enforcement, and whether the private anguish of parents who have lost sons and daughters in alcohol- related accidents leads to sound public policy -- are far from settled.

What does seem clear from conversations with law enforcement officials is that police departments, particularly in the Washington suburbs, believe that they have what amounts to a mandate from politicians and the public to do something -- as long as it is legal -- about young people who drink and drive.

Indeed, the Fairfax County police chief, Col. John E. Granfield, said there is "overwhelming" public support for the department's actions and that he foresees no change or formal review in the practice of sending plainclothes officers to certain large parties where there is reasonable cause to believe that the law is being broken.

"Times have changed," Granfield said in an interview last week. "The goal now is not to have intoxicated kids killing themselves and everyone else . . . . People leave it to the police to do what must be done to prevent that.

"If we didn't do this and someone got killed, the other side would say, 'Why didn't the police do anything?' "

But Judi Booe, a vice president of the Fairfax County Federation of Citizens Associations and a former chairwoman of that group's public safety committee, said, "I don't think people expect undercover policemen to crash a party. I don't think that's appropriate behavior. I find it hard to believe they wouldn't put their men in uniform and on the street as a crime deterrent and a drunk-driving deterrent."

Booe added that having undercover police at a party "kind of fringes on mild deception."

The incident that touched off the current furor took place in the early hours of June 12. Uniformed officers on patrol reported spotting several dozen teen-agers milling around the neighborhood of Robert and Retha Morgan, whose son was graduating from Oakton High School.

According to the officers, some of the partygoers appeared to be drinking or drunk, although there had been no complaint from neighbors.

Two or three Fairfax plainclothes officers and several agents from the Virginia Alcoholic Beverage Control board blended into the crowd. They had no warrant.

Ultimately, several youths were charged with underage drinking, and Retha Morgan was charged with being drunk in public when, empty glass in hand, she approached a police roadblock at the end of her street. Morgan was handcuffed, in accordance with standard police procedures, taken to a magistrate, and ordered to spend the remainder of the night in jail.

Questioned by reporters, police said they had used the tactic commonly for years.

The objective, they said, was to stop youths from drinking before they caused harm to property or life -- a "proactive" policy, in Granfield's view, in line with recent legislative efforts to raise the drinking age and toughen laws against drunk driving.

However, in subsequent interviews, current and former police officials, as well as Fairfax Commonwealth's Attorney Robert F. Horan Jr., acknowledged that undercover work at large yard parties is rare. Even members of the police chief's Fairfax Citizens Advisory Council said they were not aware of the practice.

Moreover, such undercover operations appear to be used by some district stations, such as Reston, but shunned by others, such as McLean. The Reston district station is responsible for the Vienna area.

Horan and police officials said they do not keep track of arrests or prosecutions stemming from undercover work at such parties. Nor are they aware of the rate of conviction in such cases, they said. In any event, said Granfield, the resources and manpower for such activity are limited.

How, then, is the public to measure whether such a practice is a success or a failure? What are the benefits? Do they outweigh the costs?

Maurice A. Sovern, a member of the police advisory council, acknowledged that measuring the intended benefits -- primarily fewer alcohol-related highway deaths -- is difficult. "How do you prove a negative?" he asked. "If {the police} do this and nobody is hurt, how do you prove there is a benefit? You can't, but I think it's there."

Fairfax police policy is to enter the grounds of a home that is the site of a large party, but not the house itself. Officials said there must be a reasonable indication of the potential for breaking the law -- such as underage drinking, overflowing crowds or destructive behavior -- before undercover officers are dispatched, and that authorities will not necessarily wait for neighbors to complain if a police officer in a cruiser first notices what appears to be a problem.

The police tend to see objections to this practice as similar to those aimed at laws requiring seat belt use: that is, that the state is intruding in the realm of personal predilections. But critics of the police seem to go further, seeing such undercover work as a violation of the sanctity of the home.

"I think it's a rather dangerous thing," said Thomas B. (Bo) White, former president of the county federation of citizens assocations. "I'm in sympathy with what they're trying to do, but I'm afraid they may be going too far."

One vocal ally of the police in the current debate is Mothers Against Drunk Driving, whose Northern Virginia chapter claims 15,000 dues-paying members. MADD has engaged local police departments nationwide in a dialogue on how best to curtail teen-age drunkenness on the highways.

The organization, whose core membership includes parents whose children died in alcohol-related accidents, carries a moral force virtually unmatched in local politics.

MADD's goals, said Sovern, the police advisory board member, are "socially unassailable." A politician who opposes the group "will not only be run out of office but probably be stoned in the town square," he said.

In traffic-clogged urban areas such as Fairfax, MADD has been emphatic about the need to toughen enforcement of drunk-driving laws, stressing that the sheer number of cars on the road gives drunk drivers little margin for error. MADD members are quick to point out that the number of teen-agers killed in alcohol-related highway accidents in the county jumped from three in 1985 to 11 last year.

"I think the police should be enforcing the drinking age," said Stuart Schmid, president of the Northern Virginia chapter of MADD. "I've tried very hard to understand how many parents might feel their rights are being threatened by the policies of the Fairfax County police . . . . {But} after all, {undercover police} are protecting the welfare of the kids. I don't have a problem with that."