An equal $4000/student/year is not enough to hire anyone for a special needs student, who 1) won’t be able to benefit from the economy of scale in a normal classroom and 2) has needs that require a larger portion of a person’s time to attend to.
You need many multiple times the investment to accommodate kids with special needs because they have special needs.
>An equal $4000/student/year
You're off by almost 4x the average if you live in the US. For reference, for the $~16k spent per year, I was able to (privately) hire someone to take care of my infant over 40 hours a week (and all 12 months), an infant that needed around the clock care and couldn't be counted on to go unwatched for even a few seconds and who constantly irritated others with utterly mind-shattering screaming colic.
Let's say the state spends $15K per student with no disabilities. The state says to the parent, "OK, we'll give you $25K to take care of your kid."
The parents grumble, but they find a school that caters to those kids and will take that voucher. Would it be much worse than they're getting now? I doubt it. If it is, the state can subsidize that school, and probably still end up spending less than they are now.
Now it's a question of money, as it should be for a state-wide program. Would the state say "we'll give you $150K to take care of your kid?" Probably not. Really extreme cases that need that much money could be handled by other public & private organizations, but the state gains a measure of reasonableness for the school budget.
I disagree with the rest. Universal schooling is an important part of a health society. It is up to the government to provide adequate funding for its obligations. Systematically discriminating to ease budget constraints is not an ethical solution.
If I give Johnny, Jimmy, and Karen an equal investment of $20 and 20 hours of labor each for their education, I haven't "discarded" Karen, even if she needs more money and more time to get the same equality of outcome. In the same vein, I don't at all want to "discard" special ed children, I just want any public funds provided to the other children to be a nearly equal monetary investment.
>That’s why we have laws like the ADA
The period after passing of the ADA was associated with sharp drops in employment inclusion of the disabled [0]. The ADA may have actually been one of the biggest drivers of the discarding of the disabled. Not only that, the ADA encouraged racketeering against business owners for disingenuous accommodation complaints (someone off the street runs up, asked to use your bathroom, you allow the public to use it just this once and bam ADA complaint as they were secretly working for a lawyer checking for the "right" kind of grab bar) where businesses sometimes end up closing accommodations to the public. Personally I am heavily against the ADA as I believe eliminating these "protections" helps protect the disabled's inclusion within society.
[0] https://economics.mit.edu/files/17