LEAVING LOCKE HORN by Dorothy Casey (Algonquin Books: $15.95; 286 pp.). "Leaving Locke Horn" is another coming-of-age novel, but this is the home-spun version. And no one seems traumatized. In fact, there's a real feeling of innocence about it. The locale is the fictitious mill town of Locke Haven in Maine, and the main characters are three teen-agers--a boy and two girls. Forrest yearns to be an artist in New York; his non-conformist sister Evelyn yearns as well, but is less certain of herself, and Ruth, friend of both--more than a friend to Forrest--discovers she has a marvelous singing voice. What happens to these young people and how their lives change during a crucial year provides the action, such as it is. Their families play incidental roles, but no one is definitively drawn. This is a first novel for Casey, who was born in Maine and is now a staff member at the Center for Early Adolescence at the University of North Carolina.