Most American teen-agers drink alcoholic beverages, and one-third of the nation's high school students are "problem drinkers," a team of researchers reports. Based on a nationwide survey of alcohol use by adolescents, the report issued by the Research Triangle Institute in North Carolina concluded that three out of 10 high school students are moderate to heavy drinkers. In addition, the reachers concluded that drinking among girls is increasing, that the South continues to have the highest proportion of abstainers in the nation and that heavier drinking is related to the availability of alcohol. "Those states that allow 18-year-olds to purchase alcohol have heavier drinking. And that's reflected in a growing tendency to move the laws to a higher age," said J. Valley Rachal, director of the study. But the study also showed that despite laws against minors purchasing alcohol, seven of 10 high school students said they could "usually" or "always" obtain it. "by the 10th grade, about seven of 10 can no longer be called abstainers. But it doesn't really matter whether it's 70 percent or 80 percent. The fact is that a large portion of adolescents have at least some experience with alcohol, and a large percentage drink fairly regularly." The findings are from a follow-up study of earlier research conducted in 1974. That study surveyed 13,000 students in grades 7 through 12. The present study sampled 5,000 students in grades 10 through 12 taken at random from the 48 contiguous states and the District of Columbia. In 1974, 18 percent more girls than boys were drinking at "moderate" levels or less. But the 1978 figures showed the difference was reduced to 13 percent. In addition, the 19 percent advantage boys held as "heavy" drinkers decreased to 14 percent in the latest study. "We called the teen-ager a heavier drinker if he or she drank at least once each week and had at least five drinks on a typical drinking occasion," Rachal said. The institute's social scientists called an adolescent a "problem drinker" if he or she was drunk at least six times in a year or experienced negative consequences of alcohol with friends, family, school, police or while driving