All staff should be knowledgeable about the developmental stages their students are in, not only in the Piagetian intellectual stages, but the psychosocial, physical, and moral stages of development as well so that, as within a family, we can not guarantee safety, but we can absolutely guarantee that our response to a breech will be even handed, developmentally appropriate and respectful. As must be clear, my opinion of the safety of a school is not viewed from the perspective of a child who brings a gun to school. It is viewed from the sense of community that is fostered by consistent, appropriate responses that will allow students to feel an overarching sense of safety, while still being realistic that at any moment there could be a disaster. Metal detectors, uniformed police, drug sniffing dogs all would incrementally decrease the number of guns brought to school, or violence on the playground, but there would also be a cost. The extremes of violence are not common factors in these schools, but when they do occur, swift action, without regard to social status, or narcissistic posturing is critical.
Safety is more in the everyday interaction with students, teachers and staff.
In order for us to be aware that a person is potentially violent, or is escalating, they have to tell us one way or another. It is impossible to tell when a paranoid schizophrenic will become violent, because they lack the affect that we all use to gauge what the emotional status is of the other person. Your best clue is past behavior. What has made this person escalate in the past. How many people are becoming involved, the playground monitors, his teacher, the bus driver, the social worker, the principal.