From The Economist:

Few ideas in education are more controversial than vouchers—letting parents choose to educate their children wherever they wish at the taxpayer’s expense. First suggested by Milton Friedman, an economist, in 1955, the principle is compellingly simple. The state pays; parents choose; schools compete; standards rise; everybody gains.
Simple, perhaps, but it has aroused predictable—and often fatal—opposition from the educational establishment. Letting parents choose where to educate their children is a silly idea; professionals know best. Co-operation, not competition, is the way to improve education for all. Vouchers would increase inequality because children who are hardest to teach would be left behind.
But these arguments are now succumbing to sheer weight of evidence. Voucher schemes are running in several different countries without ill-effects for social cohesion; those that use a lottery to hand out vouchers offer proof that recipients get a better education than those that do not.
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Opponents still argue that those who exercise choice will be the most able and committed, and by clustering themselves together in better schools they will abandon the weak and voiceless to languish in rotten ones. Some cite the example of Chile, where a universal voucher scheme that allows schools to charge top-up fees seems to have improved the education of the best-off most.
The strongest evidence against this criticism comes from Sweden, where parents are freer than those in almost any other country to spend as they wish the money the government allocates to educating their children. Sweeping education reforms in 1992 not only relaxed enrolment rules in the state sector, allowing students to attend schools outside their own municipality, but also let them take their state funding to private schools, including religious ones and those operating for profit. The only real restrictions imposed on private schools were that they must run their admissions on a first-come-first-served basis and promise not to charge top-up fees (most American voucher schemes impose similar conditions).
The result has been burgeoning variety and a breakneck expansion of the private sector. At the time of the reforms only around 1% of Swedish students were educated privately; now 10% are, and growth in private schooling continues unabated.
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More evidence that choice can raise standards for all comes from Caroline Hoxby, an economist at Harvard University, who has shown that when American public schools must compete for their students with schools that accept vouchers, their performance improves. Swedish researchers say the same. It seems that those who work in state schools are just like everybody else: they do better when confronted by a bit of competition.