One commenter asks what to do about the kids who are harder to teach? This is a very good question, to which the obvious answer is "subsidize them". We already do quite a bit of this; a friend with an autistic son has the city of New York paying for his schooling because the city can't provide adequate services.
A diagnosable disorder should bring a bonus on your voucher. Of course, this means that you'll need a vast special ed bureaucracy to set standards and diagnose. But we already have it, because school districts get extra money for special ed. I'm not sure how much new administrative overhead this would entail.
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