Earlier quoted context omitted.
It is both a loophole and the main point. It's the main point for people who want their children to succeed at the expense of the rest of society and it's a loophole when they try to sell that concept to the rest of society as "we can teach better for cheaper so give us your tax money and let us replace universal public education" rather than "we have thrown some of your children on the scrapheap for you to expensive…
Please also consider: removing the one or two severely disruptive students from a classroom results in drastically better education for the 20+ students who remain, which is likely a benefit to society at large. Based on my own education, a public school where every single day the same 1 or 2 students disrupted the lesson, assaulted nearby students and forced the teachers to waste hours on maintaining order rather th…
As the root comment pointed out - we already know that you improve outcomes by excluding low performers. Schools that can be selective take advantage of that. If you are in a position to attend one of those restrictive schools it may improve your outcomes!
However, the question of "how to do a good job educating kids" on a national level must include everyone in the measure. It might be worth having schools for troubled kids! But you have to look across the results of the troubled kids and the more normate ones. I think the root comment's point is that...this seems to have copied the techniques that charter schools use to improve their educational outcomes...but that arguably those schools are not "solving" the same problem as public schools and comparing them does not reveal any insights.