Complex sentence: The Eight Schools Association (ESA) is a group of leading private college-preparatory schools in the United States, begun informally during the 1973-74 school year and formalized in 2006 with the appointment of a president and an executive director.
Split into simpler sentences: The Eight Schools Association (ESA) is a group of leading private college-prep schools in the United States. It began informally during the 1973 ‑ 74 school year and was formalized in 2006 with the appointment of a president and an executive director.