(Guest Post by Matthew Ladner)
Having learned that one-third of private schools in Indiana participate in the scholarship program, I decided to check the stats on the Louisiana program. Sure enough right on the front page is an announcement that less than one-third of Louisiana private schools participate.
Indiana and Louisiana share some broad similarities in program design- mandating of the state test, grading private schools A-F, etc.
By way of comparison, about 71% of Florida private schools participate in the Florida Step Up for Students program. The Step Up program has a provision for testing, but allows the private schools options in which test to take. The state sponsors rigorous evaluations of the program and does apply rigorous financial accountability standards, but is otherwise largely content to allow parents to serve as the arbiters of the fit between the school and the student.
There are no right or wrong answers here, but it may also be the case that provisions like those in Indiana and Louisiana may come with a rather large cost in terms of broadening options for parents.
Jay P. Greene's Blog 
[…] its tax-credit scholarships, every private school but three participates. To this point, 71 percent of Florida’s private schools have participated in its scholarships. Florida’s program is a little clunkier than Arizona’s, […]