Are School Vouchers Really Bad?

A study from the United States Department of Education shows that after two years, K-12 students who availed of scholarships under a federal voucher program in the District of Columbia are performing about 10 percentile points lower in a standardized math exam than students who were not in the program. Julian Hellig at Cloaking Inequality uses some humor in relating the above results. He adds famous "fake quotes" inspired by Britney Spears' 'Hit Me Baby One More Time':

“Oh baby, baby, how was I supposed to know” Howard Fuller
“I must confess I still believe (still believe)” Betsy DeVos
“The reason I breathe is you” Mike Petrilli
“There’s nothing that I wouldn’t do” Donald Trump
“It’s not the way I planned it” Matt Chingos
“Don’t you know I still believe” Rick Hess

It is true that students under the voucher program are scoring lower than those who are not, but the correlation between vouchers and poor performance is certainly not generalizable, and the reason behind the scores is not really clear in the study. 

Above copied from
EVALUATION OF THE DC OPPORTUNITY SCHOLARSHIP PROGRAM 

The authors of the study do cite differences between the schools attended by the students, but there is nothing among these factors that can explain the difference in performance. 

There is, however, a trend that is worth noting. During this study, scores in mathematics are actually improving for all students in the District of Columbia. The authors of the study are thoughtful enough to remind us, "It is important to note that students in both the treatment and the control groups scored higher on the tests two years later than they did at the time of application. The impacts were negative because the gains in test scores for the treatment group were smaller than the gains in test scores for the control group. An analogy is to a footrace—all students are running forward but the control group students are running faster."

With this trend in mind, one should therefore consider the fact that the control group has also experienced a change in this two-year period. When a student takes a voucher to enroll in another school, it means one less student in the struggling public school. Thus, with the vouchers, the pupil to staff ratio can be significantly lowered in the public school. Of course, lower pupil to teacher ratios can possibly mean more attention is not provided to each student. The decrease may not be gigantic, but for someone who has been teaching for decades now, near a certain threshold, having one more or one less student can be significant.