Caroline Hoxby, economist at Harvard University, explains the effect of competition on school performance. She writes:

Public schools do respond constructively to competition, by raising their achievement and productivity. The best studies on this question examine the introduction of choice programs that have been sufficiently large and long-lived to produce competition. Students’ achievement generally does rise when they attend voucher or charter schools. The best studies on this question use, as a control group, students who are randomized out of choice programs. Not only do currently enacted voucher and charter school programs not cream-skim; they disproportionately attract students who were performing badly in their regular public schools.