This report is from the Monitoring the Future survey (MTF) which is given every year to measure drug, alcohol and tobacco use and related attitudes among 8th, 10th and 12th graders.
Marijuana use by adolescents declined from the late 1990s until the mid-to-late 2000s, but has been on the increase since then. In 2013, 7.0 percent of 8th graders, 18.0 percent of 10th graders and 22.7 percent of 12th graders used marijuana in the past month, up from 5.8 percent, 13.8 percent and 19.4 percent in 2008. Daily use has also increased; 6.5 percent of 12th graders now use marijuana every day compared to 5 percent in the mid 2000s.
Historically, as perception of risks goes down, use goes up (and visa versa). Young people are showing less disapproval of marijuana use and decreased perception that marijuana is dangerous. The growing perception of marijuana as a safe drug may reflect recent public discussions over "medical marijuana" and movements to legalize the drug for adult recreation use in some states.