I bet extrapolating that example over the whole universe would show there is little diviation to your observation. This is why I am a proponent of school choice. The parents of those 15 who obviously want their children to do well are being screwed by a system that just wishes to warehouse kids under the onus they need be educated till they are 16 or 18 depending on the state. When I was a child, 60 yrs ago, you acted up or got in legal trouble, off you went to reform school where you were perhaps taught a trade but you were removed from the general student population and ceased to be a disruption. Maybe that is the answer today but in reverse, let the good students go to a place where they can learn and leave the rabble in their inner city holding cells till they either reform and move on to prison.
"School choice" schools generally fall into two groups. The first group consists of "charter schools", chartered by the government. These schools have the same problems as public schools because of government interference.
The second group are the truly private schools. I taught in one of those for a number of years. They do tend to self-select, and so fragment society. That's not good.
However, most urban public schools are well beyond saving. It's worth the risk to allow true school choice there.
From reading Robert Weissberg's book "Bad Students, Not Bad Schools" many or all of those parents and their children have the option to send their kids to some alternative i.e. charter school. Most turn it down.