A national survey of 49,500 junior high and high school students conducted in the spring of 1991 and released late last month shows that by the eighth grade, 44 percent had experimented with cigarettes, and 14 percent had smoked within the previous 30 days. Among high school seniors, nearly two-thirds had tried smoking and 28 percent had smoked in the last 30 days. Survey researchers said those numbers underestimated the real proportions of children smoking because they did not include dropouts, whose rate of smoking in other surveys has been as high as 75 percent.