Even if we assume that's the case, that's not the problem.
The problem is that the school system fails to provide the necessary and sufficient services that would prevent a statistically significant number of 3rd graders from being held back. Feeling the need to hold kids back is a symptom of the problem, not a solution.
I strongly suspect that Mississippi should be allocating more resources to education. But this is a political problem and the schools have nearly no say in whether the legislature does or does not increase funding.
So. Do we close down the schools and wait until it is resolved?
Or do the schools do the best they can with the resources they have? Do you have evidence that placing kids in the most skill-appropriate classroom is a worse use of available resources than placing them in the “correct” classroom based on age or previous cohort?
"Ton of spending" are weasel words. "Not a ton of results" is already the problem.
If your school system fails to teach kids how to read after 3 years, this is a school system that fails at it's primary and most basic responsibility. These third-graders are not the problem, they are the canary in the coal mine.
Advocating for holding back third graders and expelling underperformers is a kin to advocate for getting rid of canaries because they are a nuisance when assessing health and safety.
I never disagreed on this point.
Now what? Every morning, kids wake up a day older. Is there a way to hit pause so you have time to go in and fix it?