There are two opposing issues here in that if you are trapped inside, you need to hide from the shooter while at the same time...
Make sure you keep your hands in plain sight at all times to minimize your chance of getting shot by responding officers.
I don't work in an office with cubical-mates, but if I did I'd try to be like the first group that got out in the video. Some things you discuss ahead of time so that when the time comes you don't talk at all, just get up and go, no fire alarm required, Time to Go.
Good video if for no other reason than prompting discussion.