Complex sentence: Fairfield Country Day School is broken down into two distinct student bodies: a lower school and an upper school, the former defining all students in grades K-4 and the latter defining those in grades 5-9.
Split into simpler sentences: Fairfield Country Day School is broken down into two distinct student bodies: a lower school and an upper school. The former is defined as all students in grades K-4 and the latter as those in grades 5-9.