That marijuana is classified as a "schedule I drug" is, itself, obscene and a result of politics, not science.
Therefore, the rest of your suggestion regarding them "playing with fire" does not apply in this instance.
See: http://www.time.com/time/health/article/0,8599,2005559-2,00....
The only real reason is that 90-70 years ago, the white people smoke tobacco and the black people smoke marijuana.
Even if Alcohol doesn't specifically make schizophrenia worse, alcohol abuse is the most common comorbid disorder of schizophrenics and generally exacerbates all mental health problems to some extent.
Alcohol use disorder, which is three times more common in patients with schizophrenia than in the general population, and cannabis use disorder, which is up to 10 times more common, both contribute to the morbidity of schizophrenia, through increased relapse, noncompliance with treatment, more hospitalizations, and poorer overall functioning.
Does cannabis have a causitive or trigger effect on schizophrenia? Or are schizophrenics more likely to live on the fringes of society and encounter (and like) cannabis?
However I think a fairly safe default is to tell kids not to smoke pot until we can make a more educated decision (provided of course we don't extend that ban to adults for no reason, and provided we don't purposely ruin the lives of kids who don't listen. (You wouldn't arrest or kick a kid out of school for smoking tobacco.)).
I think it's related to the whole paranoia side of things.
More bluntly: because children are screwed up faster by the collective parenting habits of multiple generations of mentally ill ancestors, they turn to possible fixes outside of their screwed up families: junior high + pot is normal. Marijuana, if anything, is a minor symptom here.
Now, if you want to say "playing with stimulants" or "playing with depressants" or some other scientific classification, then that makes sense (though I would still like to see a citation..).