Drums of Courage by Thomas Linehan
Thirteen year old Cory Cromwell from a small town 20 miles north of Philadelphia is angry about his deceased father, and frustrated by a school bully. He convinces his distraught, widowed mother to approve his joining the Union Army as a drummer boy. Cory discovers his nemesis, the bully, follows Cory into army life and continues to torment him. Cory soon learns that a far more serious foe is stalking him.