PHILADELPHIA — The voice came, loud and insistent, over the loudspeaker: “There’s a lockdown, there’s a lockdown.” Christian Kelly, a 12th grader sitting in his classroom, felt his heart race. On this sunny Thursday afternoon, gunfire had erupted across the street from the North Philadelphia U School. Kelly was inside, but some of his classmates were in the schoolyard, and others inside the corner store across the street, inches away from the shooting. Adults on the playground sprang into action, hustling inside the 10 or so students who watched the shootout unfold — but not before they all wa…