The original purpose of this study was to test several types of anti-cannabis messages. Analysis of the results led to a second, post hoc purpose: to explain why pre-intervention substance usage rates varied so greatly between the sampled schools. Participants were Australian ninth-grade students at a state government high school and a non-denominational Christian high school. Analysis of the open-ended responses indicated that attending a school that has a strong moral prohibition on substance use--reinforced by peers, families, and church--acts as a protective factor in preventing substance use. Studies such as this suggest that providing adolescents with supportive environments will encourage them to make sensible choices about drugs, with long-term benefits for the individual and the community.