“The Colburn School, where thousands of Southern California children have learned to master a musical instrument,” has a big, new building, a new president, 100 live-in students at its Colburn Conservatory of Music, and a fight on its hands. “Parents of the 1,500-plus kids attending the Colburn’s popular community-based School of Performing Arts division say they’re being shortchanged as attention is shifting to professional training for the conservatory students.”