In 1980, for the first time in 20 years, the number of high school students smoking marijuana declined, social psychologists at the University of Michigan reported last week. However, there was an increase in the number of students using some drugs, including stimulants and the sedative methaqualone, the researchers at the university's Institute for Social Research concluded. "Overall, I would say that we are observing a gradually increasing conservatism among young people in their attitudes toward drugs," said Lloyd Johnston, one of three social psychologists who compiled the study. "We still have a long way to go before we return to anything like the relatively drug-free years of the '50s," Johnston said. "In fact, it seems unlikely that we ever will return to those levels. But the dramatic rise over the last two decades in the proportion of young people involved with illicit drugs appears at or very near an end." The researchers compared nearly 17,000 seniors in the class of 1980 with seniors from previous classes. They found that only 34 percent of the students had used marijuana in the month before the 1980 survey, compared with 37 percent the year before. "Of perhaps greater importance, the proportion of the age group who frequently use marijuana also has been declining," from a high of 11 percent in 1978 to 9 percent in 1980, the researchers noted. The researchers believe peer pressure is one reason for the decline. In addition, medical studies have found new health risks associated with marijuana use. . . .