First, be wary of "high performing" schools — the best schools will show a nice, linear gain in test scores from year to year. A 10-20 percent jump should make you question scores' validity.
Second, classrooms increasingly include a mix of student abilities — a concerned parent might do well to look for a classroom with a more restricted range. (Warning: massively controversial.)
Finally and most importantly, my Ed Stats sources point to a Harvard/Columbia study of teachers' value added. A good teacher is HUGELY important, but currently no metrics exist to *predict* teacher quality — you can only see it in hindsight.
The best thing a parent can do: until an objective measure of teacher quality crawls from the Petri dish, gather *subjective* information about your child's potential teachers."
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