I don't think I'm ignoring the harm. I am accepting it. We can get into the utilitarian calculus, but before even considering that I don't think it's acceptable to force students who want to succeed into classrooms with those that don't. And really that's the end of the story for me. Maybe the total outcome is worse because the kicked out students cause much bigger problems than they would otherwise but that doesn't mean we should force the other students to suffer. I don't feel right dooming those kids to a poor education.
The public schools do deserve blame for putting all the under privileged kids together. The charters deserve credit for allowing them to separate themselves. I don't think that's intrinsic to public schooling, it's just the circumstance we are in.
> I don't think it's acceptable to force students who want to succeed into classrooms with those that don't
Now you're ascribing a specific cause and moralizing, assuming that it's harder for some subset of students because they want to fail, and that they deserve to fail. The utilitarian calculus does matter, you feel fine dooming some set of kids to a poor education because you think they deserve it.
No child deserves a poor education. We do have to choose who we prioritize and I don't really think your analysis of who "deserves" more help is sound.
The public schools do deserve blame for putting all the under privileged kids together. The charters deserve credit for allowing them to separate themselves. I don't think that's intrinsic to public schooling, it's just the circumstance we are in.