On the day 5-year-old Lens Joseph was killed by a Boston Public Schools bus last year, the driver had already struck a postal truck, ignored a stop sign and missed several stops, prosecutors said. When he got to Lens’ house, he dropped him off on the wrong side of the street and then ran over the kindergartner as he crossed in front of the bus. Transdev, a multinational company that has been the city’s sole bus contractor since 2013, hired and trained the driver of the bus that killed Lens.