I think competition shouldn't be taught or supported in schools. Society (aside from the educational *experience) promotes it enough. I'm not talking about athletics or personal physical development.
Private schools may show a better scoring student, but IMHO, one who has a either a lot of unresolved social issues or no *reason to develop further. It's hard to generalize, as you know.
Voucher systems, if they are to make sense, should then allow public schools to *start charging (the same amount of) tuition so they may work from or at least begin working toward the same playing field. Once that playing field becomes *level (ha) there will be no need for 'voucher' systems.
What do you feel the 'voucher' represents?