no they haven't , else we wouldn't be having this discussion
Also, while were on the subject, you want to know the REAL reason private schools do better than public schools?
Ok here it is
As a public school, you have to accept EVERY child that lives within your district. Doesn't matter if that kid comes in kicking and screaming and declaring every day that he doesn't want to learn, he has a right to be there. Doesn't matter that they are too handicapped to take advantage of any learning situation, they have a right to be there. Doesn't even matter if they can communicate with the teachers or not, they have a right to be there.
So you invariably have kids who neither they nor their parents give a shit about education and you have kids who's parents are essentially using the schoool as a baby sitter for their handicapped child, and ALL those kids are
A) Sucking up resources that could go towards educating kids who want to learn
and
B) being figured in the GPA averages and test score averages bringing the numbers down lower than they would naturally be without them
Meanwhile of course private schools first of all are expensive and so only families serious about education usually send their kids their and second of all can simply turn away kids who can't meet whatever standards they want to set.
So logically you can see that vouchers would start bringing the less able or willing students into private schools lowering their averages.