Here is the plot summary from IMDB:
Propaganda film that relates the story, as told by high school principal Dr. Carroll to parents at a PTA meeting, of the scourge of marijuana. The tale revolves around Mae and Jack, accomplices in the distribution of marijuana, who manage to entice the local high school kids to stop by Mae's apartment to smoke reefer. The lives of all who are involved with this menace are inevitably shattered. One youngster becomes so addicted to the killer weed that a judge orders him to be committed for life to a mental hospital! Dr. Carroll advises us to not incur the same tragedy.
According to this article, the notion that pot can makes you crazy is not far from the truth. In it, researchers claim that excessive smoking to pot in your youth, leads to a break down of you mental health in later years. People who are already suffering from mental illness seem to be most at risk.