Run*. Hide. Fight. As soon as you hear the shots that tell you that 5-20 of your classmates/colleagues/fellow shoppers are in the process of dying, be sure to employ one of these options.
They left out the hardest but most effective option: regulate.
*All the other kids with the pumped up kicks
You better run, better run outrun my gun
All the other kids with the pumped up kicks
You better run, better run faster than my bullet
