If the system needs fixed, then the legislature should address some of
problems you've mentioned. A tax credit does nothing to attempt to
fix these concerns. I have a sister attending North Idaho Christian
School in Hayden Lake and I'm positive my parents would be tickled
to receive a tax credit kickback even though it would not affect their
decision of continuing to send her to a private school. Essentially
this tax credit serves as an end around for private schools to reach
their mitts into the public coffers without having to follow the
guidelines of public policy formed under the constraints of the
constitution of the United States.
The proper thing to do with this extra tax money, if it's not being
used for its intended purpose of financing public education, is to kick
it back to the taxpayers who have ponied it up in the first place.