Nearly one out of every 10 teenagers in Fairfax County And Falls Church has an alcohol-related problem and nearly one-third of the students at a high school in the suburban county drinks alcoholic beverages at least once a week, according to two surveys released yesterday.

The reports, said Fairfax Board Chairman John F. Herrity, confirm the belief of school authorities that "alcohol has replaced narcotics as the No. 1 problem in the schools."

"It has been evident to us in the police department that alcohol use and abuse among teen-agers are on the rise," agreed Fairfax County Deputy Police Chief Kenneth R. Wilson. "We've observed large parties where youngsters were carrying kegs of beer and we're even finding more alcoholic beverage containers on school property."

One of the reports, released by the Fairfax-Falls Church Community Service Board, said an estimated 4,327 youngsters between the ages of 13 and 17 in the two localities suffer from problems related to the use of alcohol. The report defined alcohol-related problems as a youngster's inability to attend school or work or suffering some other disability as a direct result of drinking.

Another survey, of 1,977 students conducted last week at Fairfax's lake Braddock High School, showed wide-spread use of alcohol among students there. Only 21 percent of students said they never drink, according to the survey.

"They're about a week late" said State Del. Warren E. Barry (R-Fairfax), who last week saw legislation die in the General Assembly tht would have raised the age at which Virginians can purchase beer.

"Had the report been released earlier, Barry said he might have had more support for his measure that would have raised the-beer-purchasing age to 19 from 16 years.

But Barry, who has been arguing that teen-age alcoholism is a major problem in Northern Virginia, said he was surprised by the numbers of teen-agers that the surveys said called themselves regular drinkers.

"Young people are returning to the establishment's use of alcohol," Barry said. "Parents accept their kids coming home a little boozed rather than messing with drugs."

The Lake Braddock student survey seemed to confirm Barry's point. It said 34 percent of the students polled obtained their alcohol from family members. Twenty-three percent said they got drinks from friends under the age of 18 and 22 percent said they obtained their liquor at parties.

There seemed to be no dispute among students that drinking, often to the point of being "drunk," was widespread. About one out of every five students polled said they had been under the influence of alcohol while at school and 27 percent of the students said they would "drink to get drunk" once a week or less. Almost half of the students said they would never drink to the point that they were intoxicated.

A similar study last year by the school newspaper at Robert E. Lee High School in Springfield found widespread drinking. Of the 400 students interviewed there, nearly three out of four said they drank alcoholic beverages. Forty-two percent of the students said they drank weekly and 5 percent said they drank daily.

"When it comes to drinking, it's caused by peer conformity rather than peer pressure," said 17-year-old Scott Gould, an editor on the student newspaper, yesterday. "It's like going out with the fellows to have a beer."

"It's so east to buy beer," said Gould, who said merchants rarely demand proof his age.