Complex sentence: He soon became an all-around athlete at Sumner high school who chose basketball as his favorite sport, although he failed to make the basketball team at Sumner instead he lettered in gymnastics and was the basketball team manager.
Split into simpler sentences: He soon became an all-around athlete at Sumner High School and chose basketball as his favorite sport, although he failed to make the basketball team at Sumner. Instead, he lettered in gymnastics and was the basketball team manager.