It started with rumors, a love triangle, and a dirty look in a high-school bathroom. Soon jokes about an "Irish slut" cropped up on Facebook, and a girl’s face was scribbled out of a class photo hanging up at school. One day, in the cafeteria, another girl marched in, pointed at her, and shouted "stay away from other people’s men." A week later, as the girl walked home, a car full of students crept close. One kid hurled a crumpled soda can out the window, followed closely by shrieks of "whore!" If your children had behaved like this, how would...