Time is running out. The annual concert is tomorrow, and Roberta Guaspari-Tzavaras is whipping herself and her second- and third-grade violinists through a frenzied rehearsal. "When I bow, you bow!" she barks. "I don't want to tell you to watch me!"
The 24 little musicians are gathered in the basement of River East elementary school in one of New York's tougher neighborhoods: East Harlem. At the back of the room is a sign admonishing them to PRACTICE MORE; in front is this dervish of a drill instructor, issuing staccato directives from beneath a cloud of frizzy, dark hair that seems charged...