Do voucher programs force public schools into a zero-sum game by redirecting public funds and promising students to private schools? Or do school-choice options spur healthy competition by pressuring public schools to improve?

School systems across the country have adopted policies, such as Florida’s A+ program, that reward or sanction schools depending on how their students perform on standardized tests. The A+ program assigns schools a grade from A to F, based on how their students score in reading and math. While...