Complex sentence: This school, often referred to as just the "University School", is broken up into three academic areas: the lower, middle, and upper schools, which respectively represent elementary, middle and high school divisions within the school.
Split into simpler sentences: This school, often referred to as just the "University School", is broken up into three academic areas: the lower; middle; and upper schools. These represent, respectively, elementary, middle and high school divisions within the school.