One type of terrorism usually ignored in threat analysis is the armed assault on soft targets such as schools by paramilitary terrorist operatives. Beslan is a good case study of how such an assault could succeed. This month's Homeland Security Affairs Journal has a rather frightening article by Bill Tallen outlining how such an scenario could play out.
The crux of the problem is one of time. The usual procedure in an active shooter event is to seal off the perimeter, gather intelligence, negotiate, and then mass for assault. Most local law enforcement are also going to want to wait for a Tier-1 team like the Delta Force or the FBI's HRT unit to deploy. But when faced with adversaries whose sole goal is to die in a spectacular fashion and take as many hostages as possible down with them, waiting only provokes disaster.