Complex sentence: Atwood was the school's chief instructor from the time it opened until he quit on June 10,1912 because he could make more money in exhibition flights and because he was disenchanted with fellow instructor Arch Freeman.
Split into simpler sentences: Atwood was the school's chief instructor from the time it opened until he quit on June 10,1912. The reasons given for his departure were that he could make more money in exhibition flights and he had become disenchanted with fellow instructor Arch Freeman.