Yes these are proprietary ratings. This is typically the first question we get: how credible/accurate are these scores. Since this is HN I can dive into this a bit more than with a typical parent:

-There are fundamentally two data sets in play here, a national one, NCES, that provides basic school information: name, # of students, demographics, etc. And a State data set.

- Each state administers their own state assessment tests for most, not all, grades. So for example, Massachusetts does tests for grades 3-8 and grade 10. Other states do a different mix. To assign a "score" to an Elementary, Middle, or High School you combine the grades, but there is judgement around, for example, what constitutes a middle school? Grade 6? Grade 8? It varies by school district, let alone state.

-The subjects covered are typically Math and English. Some sites, especially for high school, also test Science or additional subjects. In our case, we include ALL available tests in the score calculation. GreatSchools provides no insight into whether they do this or not. SchoolDigger used to just use Math/English but recently added all tests to their rankings.

-The "results" for, say, Grade 3 English are typically broken out by Advanced, Proficient, Not Proficient, Failing. But this, too, varies by state, some include 5 buckets.