The school-choice movement — which germinated 50 years ago in free-market economist Milton Friedman's fertile mind — recently counted its largest victory.

The Indiana Supreme Court unanimously upheld the constitutionality of the state's school voucher program. Under it, more than half a million low- and middle-income Hoosier students — and about 62 percent of all families — are eligible for state aid to help pay for a private or religious school.

This is what school choice has traditionally lacked: scale.

Since the first experiment in Milwaukee in 1990, voucher programs have been resisted by a powerful combination of interests. Teachers unions have fought what they regard as a diversion of resources from public education — while conveniently undermining a source of professional competition and accountability.

But this opposition has been empowered by the skepticism of many suburban parents, who have paid a premium to buy homes in better school districts. When educational outcomes become less connected to the ZIP code you inhabit, some property values will decline.

It is a paradox Friedman would have appreciated. Vouchers have been blocked by unions resisting market forces and by suburban parents reflecting those forces. Not surprisingly, support for school choice is often twice as high among urban residents as it is among suburban ones.

This has generally relegated vouchers to the margins of education reform, in underfunded microprograms aimed at the very poorest. The District of Columbia's scholarship program, for example, capped participants at 3 percent of the student population while increasing funding for public education. The political price of providing vouchers to disadvantaged children often has been to shield public schools from even the mildest competitive pressure.

A limited choice program is not the same thing as a healthy, responsive educational market. "A rule-laden, risk-averse sector," argues Frederick Hess of the American Enterprise Institute, "dominated by entrenched bureaucracies, industrial-style collective-bargaining agreements and hoary colleges of education will not casually remake itself just because students have the right to switch schools."

But even restricted choice programs have shown promising results — not revolutionary, but promising. Last year, a group of nine educational researchers summarized the evidence: "Among voucher programs, random-assignment studies generally find modest improvements in reading or math scores, or both. Achievement gains are typically small in each year, but cumulative over time. Graduation rates have been studied less often, but the available evidence indicates a substantial positive impact. … Other research questions regarding voucher program participants have included student safety, parent satisfaction, racial integration, services for students with disabilities, and outcomes related to civic participation and values. Results from these studies are consistently positive."

Only recently, a few innovative governors — particularly former Gov. Mitch Daniels of Indiana and Gov. Bobby Jindal of Louisiana — have decided to bring this promise to scale. The Louisiana Supreme Court will soon issue a judgment on Jindal's program. The Indiana court found that Indiana is serving valid educational purposes both by maintaining a public schools system and by providing options beyond it. And it held (as the U.S. Supreme Court ruled in 2002) that including religious schools as an option does not establish religion.

These principles have broader implication. The pursuit of the public interest does not always require a public bureaucracy. Medicare pays for services provided at Catholic hospitals. The GI bill allowed veterans to use their scholarships at religious colleges and universities. The proper role of government is to ensure the provision of essential services — not always to provide those services itself.

In the case of failing public schools, this argument gains moral urgency. Choice may not be a systemwide panacea. But it remains a disturbing spectacle when teachers unions count it a legal "victory" when disadvantaged children are returned to troubled, unsafe institutions.

Yet it is probably not the moral arguments that will prevail. The opponents of educational choice are attempting to defend the monopoly of the neighborhood school in a nation where most monopolies and oligopolies (see the phone company, the post office or newspapers) have come under pressure. Parents, including suburban parents, increasingly expect educational options such as charters, home schooling, magnet programs and career academies. Customized, online learning will accelerate the trend.

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Michael Gerson's column is distributed by the Washington Post Writers Group.