Joe Hill, a sophomore at the prep school in this small town, stared hard at the handwritten condolences that lined the sidewalk, as if looking for answers. Lowering his sight, he shook his head in loss. As authorities investigated the murder of Melissa Jenkins, a 33-year-old teacher at St. Johnsbury Academy whose toddler was found alone in her idling car Sunday night, residents here were awash in grief, anger, and a bitter sense of betrayal.