Pharus Young is the most gifted singer in the lauded choir at Charles R. Drew Preparatory School for Boys. So why do a couple of his fellow choir members whisper slurs during his — and the choir’s — big commencement performance at the start of “Choir Boy”? Perhaps it’s because the high school junior with the cheeks of a cherub, the voice of many an angel, also has at times the snappy delivery of RuPaul.