From what we're hearing out of Cleveland, there were warning signs that the 14-year-old who shot four people in his school was planning some kind of revenge. One of the other students says he warned the principal about earlier threats, and the shooter reportedly promised, "I got something for you all," after he was suspended on Monday. Whatever the motive and the warning signs, the principal called a "code blue" in the school, once the shooting started. These days, many schools have lockdown procedures, in response to previous shootings. We'll talk with an expert on preventing school violence about what we've learned from past incidents. Did the new procedures work in Cleveland? Do you have any experience with school lockdown drills? Do you think they're effective?