I'm blessed to have an administration who recognizes that I am the only licensed person, the only one with medical training/education in the building. My principal has told all staff during emergencies "Heather is the boss right now; do whatever she says or get out of the way".
I agree with others give them something to do (call home/911/etc, retrieve student's belongings, get the custodian, crowd control, flag down the ambulance if called, etc) or tell them to back up. And anytime someone is giving potentially harmful advice (tilt head back during a nosebleed), this person needs to leave the area immediately. A simple, firm "That will hurt more than help. We can discuss this later. Right now, you need to go somewhere else" seems to work well in my experience.
Debrief is a must for any emergency in any setting. Even something as simple as an emailed list of "things that went well, things to improve, and things that cannot be repeated". Definitely speak with your administration about the best way to hold a staff debrief.