Should private schools be primarily accountable to parents or to government bureaucrats?

That’s the central question the Thomas B. Fordham Institute seeks to answer in the report it released Tuesday. The institute proposes that state governments should require private schools to administer state tests to all students participating in school-choice programs, and that the results should be publicized. Any private school the state deemed “persistently underperforming” would be expelled from the choice program.

This policy is well-intentioned, but a bad idea. It isn’t supported by the evidence and would be detrimental to the hundreds of thousands of students participating in school-choice programs nationwide.

First, the evidence: It is telling that the Fordham Institute cites only one study that suggests its policy “may boost student achievement.” Problematically, one of the authors of that study has already publicly cautioned against drawing this conclusion, noting that his finding is “enticing and suggestive but hardly conclusive.”

But even if the support of that one study were not in question, it would still only be one study. And a single study, no matter how carefully executed, is not a scientific basis for policy.

By contrast, there is a significant body of evidence that school-choice programs work without excessive government regulation. Of twelve randomized controlled trials — the gold standard of social-science research — eleven found that school-choice programs improve outcomes for some or all students, while only one found no statistically significant difference. None found a negative impact. And none of these school-choice programs studied were designed along the lines of the Fordham Institute proposal.

A 2009 literature review of the within-country studies comparing outcomes among different types of school systems worldwide revealed that the most market-like and least regulated education systems tend to produce student outcomes superior to more heavily regulated systems, including those with a substantial number of state-funded and regulated private schools. In short, the evidence suggests the best form of accountability is directly to parents, not government bureaucrats.

While the benefit of further regulating private schools is dubious, the harm is clear. By forcing every school to administer the same tests, states would induce conformity and stifle diversity and innovation. This is especially threatening because it’s already happening across the country, even without mandates, through the spread of the Common Core.

Forty-five states have adopted and begun to implement the Common Core’s uniform national curriculum standards. Though the standards do not prescribe a curriculum per se, the Common Core–aligned tests create a powerful incentive for schools to teach the same concepts in the same order at the same time. This would make it all but impossible for schools to experiment with new ways of tailoring education to meet the needs of individual children — they will instead have to resort to expecting that all children who happened to be born in the same year progress at the same rate across subjects. As Professor Jay Greene of the University of Arkansas has cautioned:

Such uniformity would only make sense if: 1) there was a single best way for all students to learn; 2) we knew what it was; 3) we could be sure the people running this nationalized education system would adopt that correct approach; and 4) they would remain in charge far into the future. But that isn’t how things are. There is no consensus on what all students need to know. Different students can best be taught and assessed in different ways.

If states force private schools to administer state tests, which will now be Common Core–aligned, they will almost entirely eliminate any viable alternative education systems.

Why? Even schools that don’t participate in school-choice programs, and therefore don’t have to take the state tests, will be affected. All over the world and across history, whenever private elementary and secondary schools are eligible for government subsidies, whether directly or indirectly (via school-choice programs), the share of students enrolling in unsubsidized schools falls.

Private schools that resist such regulation will be in the minority, and will gradually be driven out of business by their subsidized counterparts (much as America’s once-dominant private schools were marginalized by the spread of “free” state-run schools). Private schools may value their autonomy, but they value their existence even more. The higher the subsidy and the longer it has been in place, the more the unsubsidized sector is diminished.

If state governments expand their authority over private schools, even the Fordham Institute will likely come to regret it. Ultimately, it won’t be Fordham’s friends whose power has been expanded, but that of the teachers’ unions and other vested interests.

As Professor Greene has warned, “Minority religions shouldn’t favor building national churches, because inevitably it won’t be their gospel being preached.”