Philadelphia Orchestra concertmaster David Kim’s appearance was to be the highlight of the year for California’s El Camino Youth Symphony, but this week, as Kim prepared to leave for Silicon Valley, one of his assistant principals in Philadelphia was in a car accident, another went into labor, and he was forced to stay behind lest the orchestra go leaderless. Nonetheless, after a few phone calls, Kim was able to rustle up a passable replacement soloist for the El Camino kids – his good friend Robert Chen, concertmaster of the Chicago Symphony.