One thing that many schools are doing, including my school, is issuing name badges. These must be worn at all times. Although this won't stop students from causing violence, it could stop outsiders from easily appearing on campus. They stick out by their lack of a name badge. Further, teachers and administrators have an easier time identifying students who are causing disruptions.
Schools can also institute violence prevention programs and zero tolerance policies.
They can pay attention to subtle and overt changes in their children. Many times there are warning signs well in advance of violence. They can watch for these and report them to guidance counselors. In Summary Worries about school violence should not hamper the job we educators must perform. However, we need to remain aware of the possibility that violence could erupt anywhere.