Complex sentence: Part of the changes at Etowah High include its transitioning from a combination county-city school system to a "city only" school, which dropped the school's enrollment from more than 1,000 in the mid and late 20th century.
Split into simpler sentences: Part of the changes at Etowah High include its change from being a combination county-city school system to a "city only" school. This change effectively dropped the school's enrollment from over 1,000 in the mid and late 20th century.