Educational funding in most of the USA works a bit like insurance and/or a healthy social system. Everyone pays in based on their means - creates a pool of resources - then kids get assistance/education based on needs.
The trick here is that a lot of the kids with the most need are from families with the least means. So if the ratio of kids who need extra help/resources goes up because rich families all pull their kids out, then the schools won’t have enough funding to cover the needs of the kids that are left.
That’s exacerbated by the fact that schools in most of the USA (and definitely TN: I live in Nashville) are woefully underfunded, and rely in fundraisers and parent support groups to fill the gap in funding so teachers can have even basic supplies. Again, if most of the affluent families leave, there will be fewer parents of means there to help fill the gap.
It’s another example of rich families wanting to be able to opt out of helping poor ones.
If every student needed the same amount of support, that would be correct. But that is not the case.
Public school districts are required by law to provide whatever is needed for a student’s education. That even includes some students beyond the age of 18.
That includes everything from academically gifted students to English language learners to special needs students who require full time, 1x1 caregivers. I’ve personally worked as a substitute teacher in some of those special needs classes.
All of those specially educators and the facilities needed all cost more than an average general education kindergarten teacher.
When parents of kids who are average of slightly above average and don’t have a lot of special needs (read: often the more affluent families) pull their kids out, the ratio of kids with more meds changes.
Again, that extra support is required by laws passed by this same Tennessee legislature.
And you also ignored the issue of voluntary parent fundraising they is the lifeblood of many schools. That’s a massive gap that is made worse when affluent families pull their kids out.