Complex sentence: Hackensack is a founding member of the Northern New Jersey Interscholastic League, along with Ridgewood, they are the only founding member of the NNJIL to remain in the league.
Split into simpler sentences: Hackensack is a founding member of the Northern New Jersey Interscholastic League. Hackensack High and Ridgewood High School are the only founding members of the NNJIL to remain in the league.