IN A RECENT commentary in these pages, state Sen. Anthony H. Williams set out the arguments in favor of his proposal to enact a sweeping school-voucher program in Pennsylvania. While he declared that "the concept of school choice is simple," he failed to make the case for why Pennsylvanians should send their tax dollars to support private and parochial schools, as he proposes.

This isn't a new issue. Pennsylvanians rightly rejected school vouchers 15 years ago because it was clear to those who studied the plan that it would bleed resources from already underfunded public schools. The public didn't want to spend taxpayer funds on private schools then, and they have expressed little interest in the concept since. Current polls indicate that roughly two-thirds of state residents still oppose such a scheme. And there's no new evidence that voucher programs have produced sustained gains in student achievement anywhere they have been tried.

So what's different now? The only thing that's changed is the political landscape. Besides a governor who backs vouchers, new majorities in both houses of our state Legislature may be more inclined to turn their backs on public education. Certainly, Gov. Corbett appears to be doing that. His proposed budget would chop $1 billion from state support for K-12 education. And those cuts would hit hardest in the state's poorest districts.

Williams claims that SB 1 doesn't create an expensive entitlement. But once fully implemented, the voucher program outlined in this legislation would divert roughly $1 billion from the state's public schools. That is hardly a "small portion" of school funding, as Williams asserts.

And what will taxpayers get for their billion dollars? The honest answer is that we won't really be able to tell. Because voucher-supported schools are private, the taxpayers who'd fund them would have no right to know how their money was being spent. Private and parochial schools don't have to open their books - there'd be no public budgets, no transparency.

Just as there would be no fiscal accountability, the voucher proposal lacks any educational accountability. Backers of SB 1 say that if charter, private and parochial schools fail to perform, the market will force them to close. But how does that serve the kids who enroll in substandard schools? They'll have lost years of their education before the marketplace exercises this accountability.

Further, private schools don't participate in the PSSA. Even when private schools administer achievement tests to measure student performance, there's no requirement that they report the results, as public schools must. The fact is that private schools do not have to reveal anything about how their students perform, and SB 1 wouldn't change that.

The reality is that Williams and other voucher proponents demand accountability from traditional public schools, but SB 1 would send tax dollars to private schools while imposing no standards of any kind by which the performance of these schools can be measured. Instead, we are asked to rely on "the market." After seeing how little accountability the market imposed on those whose recklessness plunged this nation into the recession of the last three years, Pennsylvanians are entitled to be forgiven if they aren't inclined to turn the education and the future success of their children over to the accountability of the marketplace.

And there's one other major flaw in this marketplace approach. The competition it envisions between public and private schools doesn't take place on a level playing field. Public schools must accept any and every student within their attendance boundaries. Private schools can pick who they want to admit. In the world of private education, school choice means the school gets to choose its students.

Those choices often don't include kids with special needs or disabilities, or who don't speak English, or bring any number of other challenges to the classroom. SB 1 specifically allows a private or parochial school to exclude special-needs voucher students if the school doesn't have a necessary program - a choice that public schools aren't allowed to make.

Voucher supporters are fond of saying that public schools are broken - with the implication that the proper response is to abandon the education system that's been one of the essential foundations of our democracy.

The truth is that Pennsylvania's public schools have been on a long streak of improvement. For eight straight years, our public schools have recorded gains in achievement in math and reading.

EVEN OUR lowest-performing schools are making significant improvements. Since 2002, the number of students performing at grade level in these schools has increased by 31 percentage points. In the current Quality Counts report - an annual review of school performance by the national publication Education Week - Pennsylvania's K-12 student achievement ranked seventh in the nation.

That is not a record that justifies the raid envisioned in SB 1 on the tax funds that Pennsylvania has committed to its public schools.

Ted Kirsch, former head of the Philadelphia teachers union, is president of the American Federation of Teachers' Pennsylvania affiliate.