By 1770, there was a group of American composers with enough in common to be considered a "school." The leader of this school was:

William Billings, is the right answer.

Explanation:

William Billings was the leading composer of the early American archaic form, whose efforts have grown an essential component of the American folk culture. His works incorporate anthems, hymns, songs and verses etc. His songs are noted for its musical intensity, freshness, and sincere sympathies. He was an active figure who worked promoting the singing-school legend of American folk culture.