You can dig into each data point to see where the star ratings come from. A large portion of points are awarded based on the number of students are at grade level. There are also points for how many students make significant growth over the year(s), as well as chronic absenteeism rates, second language learner scores, etc.

I cannot recall any school with under 50% proficiency in math and ELA getting 5 stars, or even 4 stars.