Assuming five races: Given that there are more than five schools, than using a school's past score performance or socioeconomic status is going to be closer to

Assuming five races: Given that there are more than five schools, than using a school's past score performance or socioeconomic status is going to be closer to their individual score than the aggregate for their race. tl;dr School is finer grained than race because there are more schools than races.