Just a few things to know:

There was a gap in data for most states between old testing and new Common Core tests (Parcc or Sbac). So great schools is often rating schools on just one year of data on a new test. Not the best info.

In CA we have School Aacountability Report Cards (SARC) which have a ton of pertinent data and info. I imatine other states have something similar.

There is also a Single Plan for Student Achievement (SPSA) which explains the school's current goals and priorities.

I know greatschools and some other sites are an easy way to look at schools but you will mostly see the data mirror the socioeconomic status of the neighborhood (which is its own problem), so if you want more info, see what docs your states require.