From the Brookings Institution:
On negative effects of vouchers, by Mark Dynarski: Executive summary Recent research on statewide voucher programs in Louisiana and Indiana has found that public school students that received vouchers to attend private schools subsequently scored lower on reading and math tests compared to similar students that remained in public schools. The magnitudes of the negative impacts were large. These studies used rigorous research designs that allow for strong causal conclusions. And they showed that the results were not explained by the particular tests that were used or the possibility that students receiving vouchers transferred out of above-average public schools.
Another explanation is that our historical understanding of the superior performance of private schools is no longer accurate. Since the nineties, public schools have been under heavy pressure to improve test scores. Private schools were exempt from these accountability requirements. A recent study showed that public schools closed the score gap with private schools. That study did not look specifically at Louisiana and Indiana, but trends in scores on the National Assessment of Educational Progress for public school students in those states are similar to national trends.
In education as in medicine, ‘first, do no harm’ is a powerful guiding principle. A case to use taxpayer funds to send children of low-income parents to private schools is based on an expectation that the outcome will be positive. These recent findings point in the other direction. More needs to be known about long-term outcomes from these recently implemented voucher programs to make the case that they are a good investment of public funds. As well, we need to know if private schools would up their game in a scenario in which their performance with voucher students is reported publicly and subject to both regulatory and market accountability. ...