From a recent study on the Florida voucher program:

We analyze the impact of the accountability system on Florida’s students and schools using a three-part analysis. First, we estimate the effect of the accountability system and the threat of becoming voucher eligible on student test score performance, both in the short-run and in the longer term. Second, we study the effects of the reform on school policies and practices. Finally, we attempt to determine if the policies appear to affect student achievement or explain the change in student performance. We find that student achievement significantly increased in elementary schools that received an “F” grade by between 6 to 14 percent of a standard deviation in math and between 6 to 10 percent of a standard deviation in reading in the first year. Three years later the impacts persist.

Importantly, we also detect specific school policy changes implemented by the schools that explain part of these increases. Specifically, when faced with increased accountability pressure, schools appear to focus on low-performing students, lengthen the amount of time devoted to instruction, adopt different ways of organizing the day and learning environment of the students and teachers, increase resources available to teachers, and decrease principal control. These, combined with other policies, explain more than 15 percent of the test scores gains of students in reading and over 38 percent of the test scores gains of students in math, depending on the model specification. As such we find evidence that schools respond to accountability pressure in educationally meaningful ways.

The study can be found here. This study confirms other studies on the same topic, see here, here, here, here, here, here, here, here and here.

Link and study via The Buck Stops Here, who has more.