It may be good to study the chaos of prior mass shooting cases. For example, here's a mall shooting with an SKS rifle:
http://www.youtube.com/watch?v=I98yPokF3ps

Someone made a good point above about the loudness of the shots. If you're not used to gun shots, then you will be likely to freeze up. If you seriously want to prepare for this type of situation, there's no way around it: you must go to a range and, at a minimum, get accustomed to the sound of gun fire. (Even the sound of gunshots through ear protection is unnerving at first.) Otherwise, you're fooling yourself if you think you can respond as planned.
_________________________
If you're reading this, it's too late.