When financial struggles forced the American Boychoir School to close two years ago, many feared the loss of a prized muscial institution that had stretched back 80 years. Very quickly, however, the Princeton Girlchoir stepped up and offered to launch a boychoir program under its auspices. Now operating as Westrick Music Academy, the combined organization features three boys' ensembles and seven girls' ensembles and is looking ahead to even more new opportunities for young musicians.