Complex sentence: She earned a teacher's certificate and began teaching when she was 16, but because Marshall did not have a high school until 1910, she did not receive her high school diploma until 1913, when she was already 23 years old.
Split into simpler sentences: She earned a teacher's certificate and began teaching when she was 16. Because Marshall did not have a high school until 1910, Debo did not receive her high school diploma until 1913, when she was already 23 years old.