High schools do it. Colleges do it. All of them. Students decline to get it
I dunno, I remember essentially being taught to plagiarize. In elementary school we'd get random assigned topics, and we were pointed to the nonfiction section, and we were supposed to spit back exactly what we read. Now, the point was to learn to use the card catalog and so on, I get that. But it's not like there was ever a speech about "you can copy IN THIS CASE because we simply want proof that you know how to look up manatees in the encyclopedia; this is not a writing class." Kids don't always learn the lessons you intend.
I know in 6th grade I did this report on black holes that was hugely plagiarized from one magazine article -- not just little snippets, but the organization, the examples, and at least one illustration were ripped off. I didn't get caught, and I remember it because a few years later I was like, "Oh, wow, that wasn't right at all, was it?" But at the time I truly had no idea I was doing anything wrong. I had a bibliography; I made it clear when I was quoting the article directly. Of course the whole report was just me paraphrasing other people -- what else was I supposed to do? It's not like I personally knew about black holes.
And in jr high, we were taught more about citations, but again, in order to give us practice at those things, we got lots of assignments where we had to have X sources, and I certainly got the strong impression that a research paper meant that we were supposed to summarize what other people had said. Period. It was kinda excting in college when profs were more explicitly asking us for our own opinions and analysis. But it doesn't surprise me if some kids, when asked to express some critical thoughts of their own, genuinely don't understand how to do so. Why'd we spend all that time teaching 'em how to copy if we didn't expect them to do it? I sure don't recall anyone teaching us how to have original thoughts.
I went to a fairly well-regarded school system and had a lot of great teachers. I think this is just a systematic problem where the cumulative effect over 12 years is quite different from any single lesson. And of course sometimes kids plagiarize when they know it's wrong, because they're lazy or stupid or whatever. But sometimes they're doing exactly what they were taught to do.