Florida has launched what has to be the most fascinating school voucher experiment the nation has seen. It is elegant in its simplicity, it speaks precisely to those parents who believe the public schools are failing their children, and it leaves in the hands of the public school system the power to put the program out of business.

The scheme (dubbed A+ Plan for Education by Gov. Jeb Bush) begins by testing all fourth-graders in reading and math. If too many youngsters in a particular school fail the test, the school itself is deemed a failure. If the school fails two years out of four, its students become eligible for up to $4,000 in scholarships to attend private, parochial or other public schools of their choice -- if the new schools accept them.

If there were no failing schools -- or if failing schools get their act together quickly -- there would be no vouchers. Pro-voucher activist Clint Bolick calls the plan "the first money-back guarantee in the history of public education."

But for all the enthusiasm accompanying it, early implementation reveals some troubling flaws. The first two schools to receive the "F" have a combined enrollment of about 860 -- all of them eligible to apply for the vouchers. Parents of 92 signed up. Does that mean the others are satisfied? That they don't care? That they couldn't afford transportation or lunch? No one knows.

Of the 92 who did sign up, only 58 actually won scholarships in the lottery that is part of the plan.

Why? In Pensacola, where the failing schools are located, only four Catholic schools and one private school agreed to participate in the plan, which would require them to enroll lottery winners who apply. (The plan also allows students of failing schools to transfer to other public schools, with the school system providing transportation. Those who transfer to private or parochial schools have to furnish their own transportation and lunch.)

More ominous yet: While only a couple of schools have run afoul of the two-Fs-in-four-years provision, 79 schools (26 of them in Miami) have their first F -- not to mention a number of D grades that conceivably could slip.

Can the state afford the $4,000 scholarships for the scores of thousands of students who could become eligible in the next few years? Would there be other schools willing to take these students, even with their scholarship money in hand?

It might also be argued that the parents most likely to take advantage of the school-switching opportunity are those parents who have been most involved in the failing schools -- doing what they can to help with tutoring, fund raising and the other things all schools need. Take these parents away and the schools become infinitely worse for those left behind.

True, but who could demand that these activist parents leave their own children in a failing school when they have a chance to do better? I couldn't. Whatever the flaws of the Florida plan, it does seem to offer an escape hatch for these parents.

What it does not offer is a cure for the failing schools. Indeed, it seems by my lights to misread the failure as almost willful. The assumption seems to be that if a school is put on notice of its failure and then given a little extra money, the staff can get itself in gear and turn the school around.

My guess is that the tests used to determine which schools are good and which awful are in fact measures of the homes and neighborhoods the children come from. I believe most of the failure -- and a good deal of the success of the top-ranked schools -- happens before the children ever get to school. Opening the hatches of the sinking boat may save the lives of those children whose parents have given them water wings and swimming lessons, but it does nothing for the others.

If Florida wants to stop the school failure that is driving Jeb Bush's fascinating and innovative scheme, let it start with failing parents. Most parents, even in the poorest neighborhoods, want their children to do well in school. The problem is that they don't know how to help them do well.

So let's teach them. Let's have training programs to teach parents of preschoolers how to get their children ready for school learning. In fact, given the latest findings on brain development, I'd start years earlier than preschool. I'd start by making parent training a mandatory part of the high school curriculum, for boys and girls.

And for those who manage to escape that training, then courses in church basements, recreation centers and public housing meeting rooms for preschool and elementary-school parents. Voluntary, if that would work, but mandatory if necessary.

Even with the cleverest of voucher schemes, schools can't do it all.