In accidental shootings among teens, what percentage of them involve
drugs, most specifically marijuana?  In other words, what percentage
of the shootings could be attributable to someone using marijuana?

In a quick search for you, the only references I saw to marijuana and
shootings was regarding the trafficking of the substance.  I would
venture a guess that alcohol consumption figures in with many more
accidental shootings than does marijuana consumption.  many times,
alcohol agitates a person while marijuana usually calms a person. 
I'll continue searching and post an answer if I locate solid
references to your query.

Best regards,
journalist-ga