They were inside. They stayed there. I'm not sure how many had the opportunity to take shelter.
The simple fact is, this happened far too quickly for many to take evasive (or defensive) measures. In fact, many who could have take defensive measures may not have because of current social acceptance of passive reaction.
If anyone was to overcome this, it would have been through immediate, overpowering, and cumulative physical response. Take the guy down; regardless of the cost. Unfortunately, that didn't happen.
Some of those students who went to their 9am classes may have stayed away if they’d had prior warning. I’m not saying it would prevent the attack. I’m simply saying a few lives might have been saved.