That's the point I was making. There *are* private schools for kids who are in deep trouble, and who don't want to be in school, but they're very "special" schools. Here's an example of one in my city. Tuition is $16,000 a year.
Here is another program in my own city. The website doesn't say so, but most of the kids' fees are paid by the state anyway, because these kids are wards of the court. Most residential schools like this are *very* expensive.
I don't think that's what voucher proponents have in mind for dysfunctional high school students.
Indeed things can become complicated. I expect these failing schools are full of all kinds of students - troublemakers, motivated learners, learning disabled, and everything in between. I'd like to see vouchers come into play as a means of pressuring public schools into more effective and efficient performance while giving parents some immediate relief. It's not a simple solution either, as you've pointed out.