A great harm reduction method for drug use could be to have a teen drug ed program like we have teen sex ed programs. DARE is basically the same thing as abstinence-based sex ed, and works about as well. But a drug education program that was comprehensive and evidence-based could be incredibly helpful. If it has to be restricted to legal drugs such as alcohol, nicotine, and opioids, it can be framed as "Here are some drugs you may encounter in your adult life."
I think that "Some drugs are okay actually" and "It's okay to be an addict" are true statements, but I think it would be a very hard sell to try to get permission to teach that to teenagers, so I'll limit my ideas to what I think would be feasible to get permission for.
Some topics a theoretical evidence-based drug ed program could cover:
-Average starting dose of commonly used drugs, as well as the dose of a regular user. This helps to limit overdoses.
-Honest discussion of drug interactions and what really cannot safely be taken together, so that experimentation is limited to mixes that at least won't kill them.
-For opioids in particular, since a LOT of people are prescribed them after surgery, there should be a chart of what a good taper looks like and information about why you need to taper off of them. Even if they don't ever take them recreationally, it's important knowledge for your adult life.
-Education about how some people (Those with ADHD and/or chronic pain in particular) need to use prescription drugs regularly and how dependence differs from addiction.
-Addiction MUST be framed as morally neutral, with good explanations given of how addictions form and what you can do if you don't want one, as well as resources being freely and nonjudgmentally given for anyone who wants to break an addiction.
-Discussion of what overdose for various drugs look like, as well as a brief training on how to use Naloxone.
-Teaching them to always tell paramedics the truth when drugs are involved in a medical emergency. This includes non-overdose emergencies! If someone has an unrelated medical emergency while taking a drug, the medics need to know so they don't, say, give someone who just took an opioid more opioids because they have a broken leg.
-That it's important to keep track of EXACTLY how much you take if possible.
-The importance of having at least one trusted sober person around in case there's an emergency and you need someone who's not completely zooted, ex. a building fire.
If comprehensive and evidence-based sex ed can lead to teens having later and safer sex, I see absolutely no reason that comprehensive and evidence-based drug education couldn't have a similar effect.