It’s about forcing all students into public school when they aren’t all on the same level when it comes to intelligence or capability to learn

Huh? Don't most public schools offer various levels of the same subject for this reason? Like remedial, honors, AP, etc. In what way is this problem solved by private schools? Unless you're implying that home schooling solves this problem, which yeah sure if one of the parents works full time teaching their children and also has a high level of education themself as well as the relevant skills. Not something that can actually be scaled out across the country. And history has shown that homeschooling typically leads to inferior education.

The schools that spend the least per student have the best scores

Yeah that doesn't sound right at all. You got a source?