As a teacher, I disagree that school is beside the point. Some schools (state/private/boarding) have drug cultures among the students, and if it is normalised, then you are far more likely to become addicted. If you never go to parties/events where drugs are used, it's very unlikely you'd use. That's where it starts for most teens.
I don't know if you can say it's more prevalent in state/private now- as a lot of people have pointed out, the street prices of a lot of drugs have come down and often drugs are easier to obtain than alcohol.
In terms of county lines, in some private schools there will be students selling. Some of these students will buy/pick up these drugs from a dealer elsewhere and take them back to school with them. That's considered "county lines" or, to give it it's legal name "child criminal exploitation".
I have friends who work in private schools, and they're trained to look for the signs of this. I don't believe any of them would turn a blind eye to it, but it does happen.
(And yes, obviously, in state too).