As a follow-up to the debate that various TAPPEDers and ex-TAPPEDers and are having with respect to conservertarian claims about the efficacy of school vouchers, this from Justice Stevens's dissent in Zelman (although peripheral to the question of whether voucher programs that will cause funding to go almost exclusively to parochial schools are constitutional) seems worth quoting:
First, the severe educational crisis that confronted the Cleveland City School District when Ohio enacted its voucher program is not a matter that should affect our appraisal of its constitutionality. In the 1999—2000 school year, that program provided relief to less than five percent of the students enrolled in the district’s schools. The solution to the disastrous conditions that prevented over 90 percent of the student body from meeting basic proficiency standards obviously required massive improvements unrelated to the voucher program. Of course, the emergency may have given some families a powerful motivation to leave the public school system and accept religious indoctrination that they would otherwise have avoided, but that is not a valid reason for upholding the program.
The most obvious limitation on voucher programs, as both Matt and Ezra note, is that there's nowhere for most students to go. A market in education wouldn't function like other markets. Whereas more customers (within reason) for a department store mean more profits, more students for a school makes it harder to educate everyone, and places substantial demands on physical spaces that can't be easily expanded. Even assuming that they provide enough money for students to have a genuinely wide theoretical range of private schools to go to, which in practice is unlikely, vouchers are only an effective solution for more than tiny numbers of students if there are lots of spaces in good schools for children to go to. Or, in other words, they only work if you assume away the problem you're trying to solve in the first place. The small numbers involved and the fact that schools are very far from being like markets in consumer goods also make large transformative effects created by vouchers exceptionally implausible. And, certainly, as Matt says, to talk about vouchers in the abstract without any details about what level of funding is on the table, how we're going to pay for it, and what slots are available for students given vouchers is entirely useless.