In the PSA, “Back to School Essentials,” young students breezily show off their new school supplies before the tone veers into something much darker: A boy marveling over his new sneakers is running down the hallway not to dodge a hall monitor but a gunman. The ad reflects a grim reality for the network of survivors from the more than228,000 students who have experienced a school shootingsince the massacre at Columbine High School in 1999.