But the kids who can't get out of the crappy schools get an even crappier school.
First, you're buying the Left's assumption about education spending - that money means a better education. You assume that school districts actually need to spend all the money they spend to deliver a quality education. They don't. Keep in mind that huge amounts of "education" spending never get anywhere near the classroom. If it was about money, DC would have the best schools in the United States, and of course, the US would have some of the very best public schools in the world. Virtually nobody spends on education like we do.
Second, at least under the voucher bill I had to learn about when I worked at the Capitol, the full proportion of funding didn't follow the student. Accordingly, the per pupil funding for the public school actually went up.