Why do you think Harper Lee has Scout and Jem hear a mockingbird singing on their way to the school auditorium? What could it symbolize?

A mockingbird symbolizes innocent. Harper Lee probably has Jem and Scout hear a mockingbird as they're walking to their school alone because through the story they symbolize the innocence of youth, sheltered in their community. The action of the novel has them experiencing things far beyond what they ever have before and trying to maintain their innocence.