The three main things that a parent can do to help their child avoid drugs and alcohol, we actually call them "the big 3," they are: Have an active role in your teenager's life. Have a curfew, and have a stated expectation of non-use.
Now the first one, having an active role in your teenager's life. That's the most difficult and complicate one to accomplish over time. Let me tell you what it looks like. An active role in your teenager's life means that you are aware of what's going on in your child's school. Not the little questions, like, "What's the answer to math question number three?" That's called micro-managing, that's not awareness. Know when the play is, know when the game is, know when the mid-term or final is. Ask about the big things in your child's life at school. An active role in your kid's life is knowing who their friends are, knowing who friend's parents are, knowing the media that your child has been exposed to over the last month or two.
It also involves having an ongoing, meaningful relationship with the adults that supervise your children on a daily basis, their teachers, their administrator, their coaches, their tutors, their babysitters. All of these people have an insight into your child's life that they can share with you as long as you have a meaningful relationship with them. It's very important to maintain the relationship with them.
Active role is eating dinner with your kids as much as you possibly can. All of these things reduce the likelihood that your kids will do drugs and alcohol by massive percentages, by 50 to 75 percent, when done regularly and earnestly.
Second, a curfew. Curfews are not necessarily about fixed point in time. They are about keeping your kids in adult supervision as often as possible, or in transition to another adult supervision resumes. The issue here is unsupervised teens.
Now as your kids get older, they need a little bit more autonomy. They need a little bit more outside your adult oversight, but even kids who are going off to college need a curfew that's reasonable.
Lastly, a stated expectation of non-use. Kids do best when they have clear expectations. You say regularly, "In this family, we don't do drugs."