Depending on whether you mean Sandy Hook/Columbine type shootings or any shooting that involves any school property in any fashion then either mostly white or mostly black. If you’re averaging a school shooting a day, then you’re including any shooting involving school property so mostly black but also fewer students than you’d expect (a lot of “school shootings” are just gang violence that involves school property in some fashion and a surprising number are accidental discharge of a concealed carry that hits no one).