I was in college. One of my courses was "psychological testing." It was a history of such tests and the uses of the data generated, with a big focus on determining the validity and reliability of such measures.
One of the first things that we learned was this: the GREATEST predictor of ANY standardized test score was SES: socio-economic status. Applied to public education, it showed that parent education and income levels were better predictors of a student's, or a school's, standardized test scores than anything a teacher did.
When the "reform" movement came forward with high stakes testing at the state level, before it went federal with NCLB, I went back to my professor from that class and asked him if the tests had evolved, or if somehow those test scores could be a valid measure of schools and teachers. He debunked the reform movement's claims with statistics. Statistics that the general public and the media didn't want to hear, report, or acknowledge.
To counteract this well-known fact, the standards and accountability movement's high-stakes testing drive was promoted with the constant mantra: "We can't worry about the factors we don't control, only those we do." And then the reform moved forward with holding schools and teachers accountable for those test scores, even though the greatest factor was OUTSIDE of our control.
It was a useful gimmick/tool, and has been very successful for the "reform" movement.
These decades later, nothing has changed. The primary "driver" of test scores is still parent SES. How shifting 5 students from one school to another would affect their "achievement" on standardized test scores would be driven by...SES.
The "movement" reminds us constantly that the teacher is the biggest influence IN the building. That's true. Of course, the teacher can only do what the system allows, with the structure and resources the system gives, so that's another limiting factor. Still, the primary driver is SES.
If the nation REALLY wanted to improve school "performance," we'd be eradicating poverty.