Cannabis is “universally available” to young people in spite of draconian laws designed to keep them from it. The only thing that has been found to reduce cannabis use amongst teens is legalization, such as the kind found in Colorado, or the introduction of medical marijuana laws.
This interesting side-effect of cannabis liberalization can be seen as the opposite of the “forbidden fruit” phenomenon – that which is no longer forbidden becomes normal and boring. Simple cannabis harm-reduction techniques can easily be taught to teens, or anyone!
http://www.drugpolicy.org/resource/marijuana-legalization-colorado-one-year-status-report