Lots of variable make test scores.
In the City BOE I represented our lower grade schools often had about a 14 percent annual change of students year to year. Therefore the 6th grade classes would only have a small group of kids who had been in our system from first grade on.
We had about 12,000 kids in our system.
Our inner city community was in constant flux--we were a kind of first entry City for lack of a better term.
The English language was difficult at best for many of the younger kids and often it wasnt spoken at home much less parents being able to read it to help their kids, we had numerous bilingual ed classes. Not just Spanish as many might think but in some European languages as well. All at great cost to the Board of ed.
Tough to accurately address the quality of ed with these kinds of variable existing all the time.
A real smart kid may well be underperforming due to language issues.
I used to ask how the kids we had from beginning in grade schools did compared to the overall scores and found that the ones who we had over time when reaching 6th grade, were doing quite well even though our average scores were much lower.
As soon as kids began middle and High schools their scores went up dramatically as the parents were becoming stabilized and not moving from town to town for opportunities as when their families were younger and kids were learning English.
A guy I worked with came to America with his wife and they had 2 children -when his kids got to school age they moved back to Italy. Seems they never spoke English at home or in their neighborhood and realized the kids would have huge issues in school.
We also had a tech school and a Magnet school in town with around 3 to 4 thousand students between them. I was on the Board of the State agency running the magnet schools as well, and my sons attended the Tech school.
These are as close to private ( even though still State controlled) as my experiences gave me and were vastly different in their
academic accomplishments.
No language issues, more parental involvement and goal oriented students wanting something more mean a lot.