The principal may ask you and other personnel to assist him or her to prevent the episode. Use other professionals for support. In many cases, the presence of authority figures will preclude any violence. However, as an authority figure in the school, you must realize that some hostile acts will be directed against you, simply because you are an authority figure. Next, teachers in some departmentalized schools can take a step against violence by prohibiting bookbags in the classroom.
Weapons are much more easily concealed in a bookbag than in a notebook.
If students have lockers in the hallway for storage, bookbags can be left there, and only notebooks and texts allowed in the classroom. This may seem to be reaching a bit, but teachers who have seen a 38 caliber emerge from a bookbag in the classroom would not think so. This measure may tend to reduce the likelihood of weapons related violence in your classroom. Also, some hostage situations in public schools have taken place in the classroom, and this may help prevent those situations. Next, if practical, leave doors to the hallway open during class.