According to the 2009 Monitoring the Future study in the United States teen marijuana use tilts up, while some drugs decline in use.
History repeats itself – When the perception of the danger of using marijuana goes down, you can observe an increase in use.
The present increase of marijuana use among teenagers requires effective measures to curb use. Legalization is not such a measure.
Monitoring the Future is an ongoing study of the behaviors, attitudes, and values of American secondary school students, college students, and young adults. Each year, a total of approximately 50,000 8th, 10th and 12th grade students are surveyed (12th graders since 1975, and 8th and 10th graders since 1991). In addition, annual follow-up questionnaires are mailed to a sample of each graduating class for a number of years after their initial participation.
Text and associated tables and figures from Monitoring the Future are available here.