I don't know how many of the posters/lurkers here have ever had an LE polygraph. I had one about 15 years ago related to an issue at my place of employment. Because I knew that I was completely innocent, I agreed to a polygraph without hesitation. I now know that I would NEVER agree to something like that again, simply because it's not as straight-forward as I had imagined it might be.
In order for the person administering the test to establish a baseline, they ask a LOT of questions. Simple... where you live, names of your children, etc... and they occasionally slip in a question related to the issue that's in question. What I found troubling/disconcerting was that they asked some questions not related to the issue that I was not willing to answer with complete openness because they were very personal, and I didn't trust that the answers would be confidential.
There was no "pass/fail." But the person who analyzed the poly did tell me that my results were "troubling." NOT that they considered me to be guilty of the issue at hand, but that there were issues about which I was not completely honest. When I discussed those results with another LE person, he told me that I should probably never agree to take another test.
So, let's think about this. If these kids are taking/dealing drugs, or if they work for someplace and they're stealing bits of money from their work... or they cheated on their last final. Those are all the types of question that they might be asked - and they would be likely, I would imagine, to lie about those. So, I would imagine that the results of a poly would be unlikely to be as cut and dried as a pass/fail.
Just wanted to share that experience FWIW.