Complex sentence: In August 2011, the district opened the new Ankeny High School and Phase I of Southview Middle School, at which time the previous high school became a middle school for 8th and 9th grades.
Split into simpler sentences: In August 2011, the new Ankeny High School opened, along with Phase I of Southview Middle School. At this time, the previous high school became a middle school for 8th and 9th grades.