Its actually impossible to keep them away from drugs, its part of the culture. The thing is, what to do in those instances. Just say no is from the Reagan era and it not only didnt help, but it stigmatized it and created a whole other problem legally, but thats beyond this post.
Know they will come across it. Know that no matter their self esteem, it is uncomfortable and many times inevitable to give into at least partaking in the social aspect of the use and setting. I recommend highly the evidence based research from David Schiff on 'Clean' such as having safety text messages to be picked up and even using parents as an excuse or 'throing under the bus' such as 'I will be tested and can be thrown out by my parents', etc.
How do you have conversations regarding drugs and alcohol use? I loved the fact that it did mention that speaking honestly about the consequences specially if they are real can minimize the statistics of use later on. There is more to learn...