Hardcore opponents of gun control in the U.S. often respond to school shootings by proposing to arm teachers or add armed security guards to schools. But the inability of police to stop the gunman at Robb Elementary School in Uvalde, Texas, before he killed at least 19 children and two teachers Tuesday exposes the poor reasoning behind that proposal. While all the details of the shooting are not yet clear, authorities’ accounts revealed that the gunman was confronted by multiple armed police officers — yet they were unable to stop him before he killed nearly two dozen people. The gunman...