Complex sentence: He attended P.S.139, where his English teacher was Countee Cullen, and later, DeWitt Clinton High School in the Bronx, where he worked on the literary magazine, "The Magpie," as a writer and artist.
Split into simpler sentences: He attended P.S.139, where his English teacher was Countee Cullen. Later, he attended DeWitt Clinton High School in the Bronx, where he worked on the literary magazine, "The Magpie," as a writer and artist.