There are three primary environments that teens live in: school, friends and home. Sometimes these environments blend into one another. But a home where kids are unsupervised for large amounts of time, say in a home where the parents work long hours, boredom and curiosity can set in. At the same time, if parents are very permissive, and let a teen stay out late, or fail to ask questions and hold them accountable for their whereabouts and behavior, drinking or using drugs is more likely.
The same is true for schools with poor enforcement by teachers or administrators, where there is a lack of supervision at lunch or between classes, before or after school, or even in certain classrooms, which makes planning to use, planning to get drugs or alcohol, or actual drug dealing much easier and more likely.
Lastly, being out with friends where there are no adults is an environment, many times, where the use of drugs and alcohol is not only acceptable but encouraged. Remove the rules of parents and teachers, and suddenly the likelihood a kid will use is much higher.