Police up to speed.
A nearly impossible job in an impossible situation. They aren't supermen, just guys pulled off routine stuff and inserted into chaos trying to do the best they can, follow orders, communicate, establish command and control, get a plan and execute it, et, et. Two minutes before they were writing tickets.
The lesson is, if YOU are caught in a situation like this, you need to be thinking, planning and acting. Until help actually shows up, you are on your own.
Barricading the door was a good reaction. Going out the window was a good reaction. 911 was a good reaction. Someone may have tried to tackle the shooter and gotten killed without witnesses. Nobody will know.
These were extraordinary circumstances for all involved. Everyone tried to do their job and help.