Jennifer Smith was worried before her son ever boarded the bus. The boy, a 6-year-old kindergartner at Mather Elementary School in Dorchester, had never ridden a school bus before. But when Smith’s work schedule changed, she reluctantly signed her child up for transportation. Samuel “Sammy’’ Aaron rode the bus just once and his mother has regretted it ever since. In January, Jennifer Smith arrived at the Mattapan bus stop on time, but her son failed to show.