There IS conclusive and substantial evidence that cannabis increases your risk for psychosis (beyond the transient psychosis seen during intoxication). It's perhaps the only major negative effect that's been found so far. I wrote a whole post on this and other possible effects cannabis may have but for some reason it got deleted.
A meta analysis of 83 studies found that onset of psychosis occurred significantly earlier in patients who used cannabis compared to those who did not. By contrast alcohol was not associated with earlier age of onset.
A systematic review of 35 studies found significantly increased risk of psychosis for those who ever used cannabis compared to those who did not. They also found a two-fold increase in risk for people who used cannabis most frequently. They adjusted for confounding factors and excluded cohorts with identified mental illness or other substance use problems.
Another systematic review (albeit with limitations) found cannabis use was consistently associated with increased relapse of psychosis, rehospitalization, and poor compliance with treatment.
Someone above is also correct that genetic factors may influence the risk of psychosis in adults who used cannabis as adolescents which may explain why the vast majority of users do not develop symptoms. 1 and 2
Now I'm not saying that a person's risk of becoming schizophrenic is high, but it IS increased. Our lecturer on marijuana and drug addiction did emphasize that based on the evidence and current literature out there, if an adolescent or anyone with any family history of any mental illness, including depression, smokes marijuana, we especially should warn them of the risks and try to help them stop.