How about a pocket knife?
I carry a Spiderco Delica in my front pocket wherever I go.
At arms reach or even within a few steps, Knife>gun for a deadly weapon. From the way things sound, many of the students were 'executed' at short range. A person with a knife and even mimimal training with it, or better yet training in disarming,could have cut the guy to pieces.

It would be neat if students around the country started training Krav Maga and carrying a K-bar in their bookbags, but I'm not getting my hopes up about it.