when their 'friends' convince them to take it is when the mistake occurs. kids that age are too reliant on what their friends do and say.. HOWEVER if you as someone who they look up to and respect talk to them (and i'm talking about you as someone who walks the walk) they will listen. however if someone is stupid you can't reason with them.. if they are intelligent you can. I say this as a HS judge and coach for weightlifting who lifts in front of these kids and they look up to .. they are well aware that I did NOT touch steroids until I was almost 30. also they are aware that I was one of the top weightlifters in the state when I was their age and it was all hard work in the gym and a lot of running to make weight
You have some exceptional kids listening to you then. When I was that age, you could have shown me evidence that one push of the plunger would make me spontaneously combust and I would have brushed it off if it was something I wanted to or felt like I needed to do.
Best thing that ever happened to me was getting scammed internationally by a guy claiming to be dealing test. Saved up a bunch of money from work, sent it via Western Union and imagine that, no juice showed up. Of course I was angry at the time, I was 18, but if I could meet that guy nowadays I would give him a big fat bear hug.
Unfortunately I had to pay 300 bucks to learn my lesson. I can only hope that kids like this figure it out free