One of the popular arguments from the pro pot crowd is how harmless it is...
So counter them with facts - not scare stories about "psychosis" (much less "ODs" as one Drug Warrior was peddling on this thread). Lie to kids about the harms of marijuana and get caught, and they won't believe the truth about the harms of marijuana.
One good turn deserves another...
https://www.ncjrs.gov/ondcppubs/publications/pdf/marijuana_myths_facts.pdf
http://www.sciencedaily.com/releases/2009/08/090805110741.htm
http://www.justthinktwice.com/factsfiction/fiction_marijuana_is_harmless.html.
http://www.justthinktwice.com/consequences/drugged_driving_you_get_high_and_drive.html