In a school shooting scenario, unless you were the first classroom hit, you would likely have time to put your vest on, assuming you had it pre-adjusted and readily to hand, and didn't dilly-dally.
As far as barricading the doors, I would consider using tables stood on end and backed up by desks, not as a barricade so much as a way to funnel the shooter into a position where we could conduct a hasty ambush.
Just some thoughts.

