Could anyone have stopped this? That's one of the biggest questions for schools and educators as the nation takes in the facts of the shooting in Parkland, Fla., that has left 17 dead and 23 injured. While the U.S. remains a global outlier by far when it comes to mass shootings, and owns 42 percent of the world's guns, the fact is that most schools in the country have taken steps to prepare for this kind of threat. Since the Columbine massacre in 1999, schools have changed the way they respond to both potential threats and actual attacks. And they've done so even without coordinated federal oversight or much in the way of dedicated resources, either for training, safety or broader prevention.