This is awesome that you came to our community for these type of answers; that speaks volumes to us, as you are in a sense accepting that there is no specific way to go about speaking to kids on the subject. At least, no known effective ways.

I thought the best idea brought up would be bringing in a recovering addict.. But not who you would expect, bring in someone that was like them when they were younger. Clean appearance, good grades, college degree. Someone they could aspire to being when they grow up. Then have that (ex)addict explain how they became homeless or almost starved or froze to death in their life because of opiate addiction. Have them explain how it's a long, slow, slippery slope. How they basically were in withdrawal for years and they have barely managed to pull themselves out of the huge hole. When I was younger I always thought I was smarter than everyone else. So unless I heard from someone who I could imagine myself being I would have never listened.

Good luck to you sir.