I think the goal of moderating drug use in adolescence should be 1) harm mitigation, and 2) instilling healthy drug use habits and mindsets.

2 is particularly important. We tend to think of drugs as "bad" when talking about kids, and then turn around and have a beer, smoke a cigarette, drink way too much coffee/caffeine/sugar, or in some jurisdictions consume cannabis. So our goal shouldn't be adolescent abstinence -- rather, the goal should be setting kids up for a life of healthy interactions with drugs. In a few rare cases that will be actualized in the form of abstinence, but in most cases it will take some other form. For this reason, I tend to ignore data that doesn't distinguish between "healthy" and various forms of "unhealthy" drug use.

Interpreting this data is extremely difficult.

Your interpretation is that schools are causing increased substance abuse.

But an equally reasonable (which is to say, not at all reasonable) interpretation is that home schooled students are more likely to have a flat, legalistic, and ultimately unhelpful understanding of drug use that will come back to bite them in the ass later in life.

For example:

* homeschoolers are more strongly disapproving of any alcohol consumption, but there's no difference in opinion about smoking 1+ packs a day. Even though the former can be non-harmful or even healthy, while the latter is pretty uniformly incredibly unhealthy. Perhaps this is because they're more concerned with following laws than healthy drug use patterns?

* Late adolescent home schooled students and early adolescent home schooled students are equally disapproving of peer alcohol consumption. To me it's weird/creepy that a 13 y/o and 18 y/o would have the same attitudes toward peer alcohol consumption, and indicates that maybe the 18 y/o's opinion is more indicative of ignorance or blind rule-following than any sort of healthy attitude about drug use per se. There is absolutely nothing unhealthy about an 18 y/o occasionally consuming alcohol (or anyways, no more unhealthy than a 21 y/o).

* A lot of the questions refer to any drug use, as opposed to abuse or modes of unhealthy use. And home schooled kids tend to be equally disapproving, or in some cases more disapproving of healthy use of stigmatized drugs than of unhealthy use of less stigmatized drugs.

* Some of the related work suggests that "homeschooled adolescents engage in less substance use than non-homeschooled adolescents, although religious ties was an important moderator in this relationship". Which makes a lot of sense, given that the overall range of opinions tends to be more indicative of unscientific moralization of drug use than of health-conscious substance use patterns.

(There's also some weird effects in this data, like home schooled kids having a harder time getting alcohol but not having a harder time getting LSD...)

To be clear, I don't think this interpretation is particularly reasonable. But I also think it's about as reasonable an extrapolation from the data as your interpretation that schools are causing the gap in the data.

> public school... are anything but meritocracies

I largely agree with criticism of educational quality provided by our schools.

I don't think it's correct to blame unions and tenure though, because non-elite private schools tend to be pretty crap as well. In both cases, the solution is probably a combination of high social status and (much) higher pay for teachers. This can be paired with eliminating tenure and unions, but doing that without significant improvements to pay will only make the job less attractive and thereby decrease quality.