Complex sentence: Harper's interest in jazz music was already cemented by the time he reached Schenley High School and he was a part of the burgeoning jazz scene of 1940's-1950's Pittsburgh that produced some of the world's greatest jazz innovators.
Split into simpler sentences: Harper's interest in jazz music was already cemented by the time he reached Schenley High School. He was part of the burgeoning jazz scene in Pittsburgh during the 1940s and 1950s that produced some of the world's greatest jazz innovators.