Complex sentence: Wayzata Central Middle School a.k.a CMS was once the high school in the Wayzata district, but when the Wayzata High School was built, CMS was demoted to middle school.
Split into simpler sentences: Wayzata Central Middle School a.k.a CMS was once that high school in the Wayzata district. When the Wayzata High School was built, CMS was demoted to middle school.