Complex sentence: O'Brian first attended school at New Trier High School in Winnetka, Illinois, then the (now defunct) Kemper Military School in Booneville, Missouri; he lettered in football, basketball, wrestling, and track.
Split into simpler sentences: O'Brian first attended school at New Trier High School in Winnetka, Illinois, then the (now defunct) Kemper Military School in Booneville, Missouri. He lettered in football, basketball, wrestling, and track.