When I was in high school, a relatively uncommon but not unknown phenomenon were bomb threats called in to my Jewish school. We dutifully evacuated the building while the police searched the premises. I remember feeling more delight at a break in routine than concern. Had the student body been polled I think more of us would have guessed that the threat originated with a fellow student trying to avoid an algebra exam rather than with a terrorist.
Fast forward to the past few weeks where Jewish schools and community centers have been targeted with warning phone calls. Things look rather different from an adult perspective. I doubt they are signaling a serious physical threat. Terrorists of the past two decades such as those who orchestrated 9/11, the Boston Marathon bombing or Fort Hood have not politely telephoned warnings of their intentions allowing lives to be saved.