The 17-year-old blond high school senior from Rockville stood up yesterday in a Rockville motel conference room. "I'm Larry," he said, "and I'm an alcoholic."

Larry said he started drinking beer in the eighth grade, and soon was drinking hard liquor before going to class each morning. A school counselor eventually reported him to his parents, who sent him to a doctor. He spent three months in a treatment program and has been attending rehabilitation meetings ever since.

On the wagon for the last 18 months, Larry was billed as a "recovering youth alcoholic" on the program, as the first of 15 conferences on teen-age alcoholism began.

The conferences, to be held nationwide, were initiated by the U.S. Department of Health and Human Services (HHS) but are organized largely by the localities where they are held. They are designed to bring together people who can share their ideas about and experiences with teen-age alcoholism.

The problem has been largely ignored, HHS Secretary Margaret M. Heckler said yesterday to about 200 people from Maryland and surrounding states. "The dimensions are outrageous," she said.

About 10,000 persons ages 16 to 24 die each year in alcohol-related traffic accidents, Heckler said, while an estimated 3 million in that age group use alcohol excessively.

In Maryland, between 2,000 and 3,000 students were suspended from school because of drinking last year, Wallace Mandell, director of Johns Hopkins University's alcoholism program, said later in the day.

The federal government can help fight the problem, Heckler said, primarily through coordination and research, but to really tackle teen alcoholism, a "grass roots effort" is needed.

The persons gathered yesterday were a cross-section of those grass roots: lawyers and teachers, ex-alcoholics and academics, a Fairfax police officer and a high school principal from New Jersey. Some wore blue jeans and tatoos; others wore business suits.

Speakers generally dismissed as ineffectual isolated efforts such as advertising campaigns or information on alcohol abuse handed out in schools. Such efforts must be coordinated by all segments of the community, including businesses, they said, and be tied into one treatment program.

In the hotel's Halpine Room, Father Jake Powderly of St. Mary's Rectory in Rockville gave a talk, aided by the Nuclear Regulatory Commission's alcohol program director and Larry, the young recovering alcoholic.

Powderly was assigned to run an alcohol program within the Roman Catholic Church in the Washington area after sobering up 15 years ago. Yesterday, using some large sheets of white paper and a black marking pen, he discussed the peer pressures teen-agers face, and how young and old become alcoholics.

Genetics are considered a likely factor in determining susceptibility to alcoholism--something Heckler said federal researchers are beginning to investigate. Powderly discussed the significance of this theory and joked: "You don't have to be Irish-Catholic to be alcoholic, but it helps."

Mandell from Johns Hopkins said that about 10 percent of high school seniors say they drink every day and more than 5 percent say they drank heavily at least six times in the previous six weeks.

Other important data about teen-age alcoholism either does not exist or has not been publicized, Mandell complained. For example, he said, no one knows the number of times youths get into trouble with the law as a result of alcohol. Such figures "rarely come to our attention," he said.