Christopher Sem played flag football at Callery Park last Saturday. Yesterday, his friends wished he was on the field, running his route and dancing into the end zone. Instead, more than 200 of them gathered to play a game in his honor. On Tuesday night, Sem, a 16-year-old Lowell High School junior was killed when the car he was riding in slammed into a utility pole on Pawtucket Boulevard. He was on his way home from playing basketball, like he did every Tuesday night.