I say the best thing to do is to train and arm teachers who are fully capable and who have volunteered to protect the students if violence erupts.  Teachers care about the students and are aware of who should and should not be present. They are capable of being cautious and probably never would need to use the weapons if criminals knew that teachers in that school are trained to defend students.  The white board may help in some situation but is a money maker for the salesman.  Selling expensive things to schools that are under budget crunch for the personal gain of the business and doing nothing for the child is almost a crime in itself.