I'm coming to this party pretty late, but here's my two cents.
First, I suppose I don't know the full circumstances, but I really doubt that there's anything approaching addition here. Most kids will try alcohol or pot at some point in their lives, and for many, the first time they try substances will be in high school. For the vast majority of cases, this will mean social drinking or smoking. Not great, but very different from a full fledged addiction.
I'm not an expert on this kind of incident, especially with respect to how it works with school policy. So, the following is just opinion. Also, I will explicitly say that my thoughts in no way reflect the views of any team I have ever been affiliated with. In fact, I suspect many on those teams would actively disagree with me.
That said...
Circumstances to intervene:
- Addiction: If you actually suspect there is some kind of addiction at work, it should be your duty both as mentor and a friend to become involved. That said, my guess is that it is unlikely addiction is at work here.
- At work sessions/competitions: If teammates are using at work sessions or at competitions, the team will likely become liable to some degree. This would have huge repercussions for everyone on the team. In addition, the concept of teenagers operating machine tools or robots in any state of impairment is so terrifying, I don't even want to think about it.
- Team vs. personal: If you suspect that the substance use is becoming a thing associated with a team vs with an individual (IE, robotics kids all go out to smoke together, vs one or two happen to smoke with other friends) than you should intervene. Robotics should not become an activity associated with drugs.
- Repeated incidents: If this is an issue more than once or twice, it brings up major concerns, especially with regards to point one.
- Role models: If team role models are creating a culture where drinking is a cool thing that older kids get to do, it needs to be stopped.
Circumstances to talk to team members, but not immediately involve admin/parents/police:
- Isolated incident: If a senior has a beer for the first time with friends at a party, this probably isn't something that concerns the team too much. Perhaps if it comes to the mentors attention, it warrants a friendly conversation from a mentor about the importance of responsible drinking, but it probably shouldn't have enormous repercussions for the individual or the team. If it isn't regular or repeated use, (I would argue) it likely isn't a problem.
And that's the list. I'd argue that the circumstances where drastic intervention is not required are very specific, but also that those circumstances encompass the vast majority of cases in high school.
These situations deserve to be taken very seriously. A mistake on a mentors part can have dire consequences. We certainly don't want valued and loved team members to destroy their lives because no one thought it was there place to intervene. As many of us have, I have had personal experience with people going down this path. On the other hand, as a mentor you could essentially ruin a teenagers future by reporting them for what might just be one mistake. Scholarships and admissions offers would likely disappear, they might lose their job, and they could be kicked out of school. It is your clear duty to consider whether or not this is in their best interests.
I don't envy your decision. It's easy to draw black and white lines on the internet, and hard to deal with grey areas in real life. But I beg you to consider the consequences your decision will have, either way. This could very will effect their entire future.