If there are no GreatSchools Ratings for a school, the school's test results were either not reported or incomplete. The different student groups are identified by the Tennessee Department of Education. If there are a small number of students in a particular group in a school, the state doesn't report data for that group.
Keep in mind that when comparing schools using GreatSchools Ratings, it's important to factor in other information, including the quality of each school's teachers, the school culture, special programs, etc.
GreatSchools Ratings cannot be compared across states, because of differences in the states' standardized testing programs.