Undercover policemen impersonating students are being used in Fairfax County high schools to help stop the sale and use of illegal drugs on school property, school and police officials said yesterday.

Undercover agents who police said look young enough to pass for students are enrolled "as a last resort" in county high schools where police and school officials determine that normal law enforcement techniques are insufficient to stop drug traffic, according to an announcement made jointly by the school system and police.

County Police Chief Co. Richard A. King said "one or two" undercover agents were placed in the school this year.

King and School Superintendent S. John Davis appeared at a press conference to announce the use of the agents as part of a new drug prevention program that will beef up drug education in county intermediate and high schools and impose stiffer penalties for students using or distributing drugs.

No other major school system in the Washington area has announced a program of using undercover police in school. School officials in Montgomery County, Arlington and Alexandria said there are no undercover agents used in the schools there. An inquiry of the District of Columbia school system drew a "no comment," and the Prince George's County school system said it "has in the past and may in the future" permit agents to work on school property.

Davis said there has been no drastie increase in the past two years in drug abuse in public schools, but that the schools have become a center for student drug trade. "Students who wish to obtain drugs often find that they are more readily available at school than on the street," Davis said.

Police released guidelines for the undercover agents yesterday. They prohibit undercover agents from posting as teacher or members of a school's Professional staff. The guidelines also said the agents "will not be used for the surveillance of individual students or student activities" other than those considered drug-related.

Davis and King said there is a possibility the presence of agents in schools could make students wary and distrustful of each other. "That is always a chance we take," Davis said. He said the decision to use agents was a "difficult one to make" and that it was necessitated by the relative ease with which students are selling drugs in schools.

Officials of the American Civil Liberties Union in Northern Virginia and Washington yesterday called the use of undercover agents in schools "appalling" and "outrageous." Lauren Selden, chairman of the Northern Virginia ACLU chapter, said, "The greatest effect it will have is that it will teach them the government can use young people to keep watch on other young people."

Davis said testerday Fairfax schools and police announced the undercover program because "we are coming out front. We don't want ot trap students. We just want to stop the use of drugs."

Police Chief King said undercover agents cannot do the job of stopping drug abuse alone, and that police must also depend upon students who do not use drugs to help catch those who do. He said that no student will be paid by police for drug-related information.

King said there always has been considerable distrust in county schools among students who use drugs towards those they consider "finks" or informers. He said there was nothing dishonorable about students who help police catch drug users. "If we didn't spu occassionally, we might not have the democracy we have today," King said.

Part of the drug prevention program announced yesterday calls for the automatic suspension for at least five days of any student found guilty of possession, use or exchange of an illegal drug or alcohol. The new regulation requires all offenses be reported to police and the student's parents.

The drug prevention program also establishes a training program to teach two faculty members from each intermediate and high school about drug problems. Davis said there is a "great deal of nivete" by adults about drugs.

Davis and King said principals at all Fairfax County high schools approve of having undercover agents on high schools.

At Fort Hunt High in the eastern part of the county, Dr. James E. Maning, the school's principal, said enrolling police as students is an "excellent" idea that could stop drug dealing on school property.

Manning said he has epotted a recent trend among more and more students to use drugs in school. "Many impression is that they think nothing will happed to them. They seem to smoke marijuana or hashish like kids 20 years ago used to smoke a cigarette," he said.