Complex sentence: Northwestern once served as a host school for the districts' Middle School & High School Chorus Festival, and it is currently used as a secondary site for the "Superiors Concert" which is the district festival for all choirs rated superior during county assessments.
Split into simpler sentences: Northwestern once served as a host school for the district's' Middle School & High School Chorus Festival. It is currently used as a secondary site for the Superiors Concert, the district festival for all choirs rated "superior during" county assessments.