From Alex Tabarrok:
Thus, this paper shows two things. First, that the test scores of the students in the public schools improved when vouchers gave the schools better incentives to perform. Second, at least some of the improvement comes from changes in how students are taught. The author's note, for example:...we find that schools receiving an “F” grade are more likely to focus on low-performing students, lengthen the amount of time devoted to instruction, adopt different ways to organize the day and learning environment of the students and teachers, increase resources available to teachers...
It is not true that "nothing can be done to improve the schools." Incentives matter.