I don't claim to have a complete answer to your question, MON, but I'll take a stab at it...
When I pulled DS7 out of public school, my former public schoolteacher mother initially had several problems that she brought up with me:
1) involved parents who care about education leaving the schools;
2) less of our tax money going to our local school, since funds are distributed based on a head count; fewer students = less money to that particular school;
3) losing the "smart kids" so that there's an even greater dumbing down of curriculum, so it's a downward spiral.
I can tell you that #2 is a SERIOUS concern for the schools. I pulled DS7 out for homeschooling during what happened to be head-count week. He wasn't out for even an hour before I was getting calls from the truant officer that DH and I would be arrested if I didn't get him into school immediately.
(*Sigh* It was my mistake to be honest about why DS was out while I was delivering the notification to homeschool instead of just lying and saying he was sick!

)
But the truant officer himself told us that they would not have been so antsy about it if I hadn't picked that particular week to pull him out. The school lost money because they couldn't count him in. So there is a real financial loss from lost students.
Of course, if they're spending the money on the kids, it shouldn't matter much, right? Fewer kids = less $$$, but it also = fewer kids to spend the money on. Yet we all know that's not really how budgeting works in practice...