If kids are ranked, even informally, nobody wants to share resources. And if rankings are real and affect outcomes further down the line, then people are much more hush hush about it.
Also, there is the phenomenon where a school gets a reputation for being "good" because of high test scores, but it is all because of the work that parents do on the side. The academics in the classroom are weak, which surprises newcomers to the school who are unaware of the secret supplementing.