Beloved groundhog Punxsutawney Phil was not the only one to see a shadow Thursday as he marked another six weeks of winter. Rather, hundreds of thousands of workplace mentors across the United States — including a significant number from the Central Valley — were shadowed by students for the day during the annual Groundhog Job Shadow Day, an initiative that has given students an up-close look at how skills learned in school are put to use in the workplace for nearly two decades. The Groundhog Job Shadow Day program began in 1996 with the first local Job Shadow Day in Boston, and a year later, the National Job Shadow Coalition was formed to encourage participation in a shadowing program throughout the country.