By Tim Reid LOS ANGELES (Reuters) – On Tuesday three students were shot and wounded outside an elementary school in Washington D.C. A day earlier, three teenagers were shot as they left their high school in Philadelphia. Last week, there were three shootings at high school graduations in Michigan, Louisiana and Tennessee. Gunfire at U.S. schools is at a record high, with a gun being discharged on a school property almost every day this year, according to David Riedman, a researcher who tracks such incidents. As Americans reeled from Tuesday’s mass shooting in which a gunman killed 19 children …