Man has this annoying tendency not to believe facts unless they are experienced directly. You can tell teens what you want and they will still do what they want, especially if they know you did the same and ended up alright.
I agree that honesty is the best policy, teenagers can smell hypocrisy from miles. Another good policy is to make experimentation safe, so everyone can make his or her mind up without risking potentially fatal or life-altering contact with the worst elements of society.
I personally think public education should also have a dedicated subject on the topic of addiction, be it to drugs, food, gambling, videogames, and whatnot. An economic system based on profit will inevitably thrive on addiction, often at the expense of public health. Surviving contact with addictive impulses is a life skill that gets more and more essential every day.
Like it or not, reality is nuanced...
>"Hey, pot is going to temporarily make you dumber. Maybe don't use it a lot while you're trying to get through school. Thanks."
You're inviting devil's advocates here though, maybe rephrase it as "pot may make you dumber". Otherwise, I think you may be providing some with a goal, something to prove. I didn't enjoy finding out my health class was dead wrong about marijuana's effects and this was certainly solid motivation to get into it at that time.
I also think we should be doing it honestly and not telling them that their brain is going to immediately leak out of their ears and they're going to start turning tricks on corners to pay for their next hit.
"Hey, pot is going to temporarily make you dumber. Maybe don't use it a lot while you're trying to get through school. Thanks."