Complex sentence: Mullin began his high school career at Power Memorial Academy, where he was a teammate of Mario Elie, but he transferred as a junior, to the all boys Catholic Xaverian High School of Bay Ridge, Brooklyn.
Split into simpler sentences: Mullin began his high school career at Power Memorial Academy, where he was a teammate of Mario Elie. He transferred as a junior to the Catholic Xaverian High School of Bay Ridge, Brooklyn.