Complex sentence: Most graduates of St. Mary's School attended high school at St. Thomas Aquinas in Florissant which later became St. Thomas Aquinas-Mercy after merging with Mercy High School in University City in September 1985.
Split into simpler sentences: Most graduates of St. Mary's School attended high school at St. Thomas Aquinas in Florissant. This later became known as St. Thomas Aquinas-Mercy after merging with Mercy High School in University City in September 1985.