As you can see, there are many methods for taking head count at the Assembly Point after an evacuation. It's the method that works for you.
At our University we have Administration Buildings, Academic Buildings, Residence Halls, and Support Facilities. Not to mention Athletic Events and other Public Events.
For Staff and Faculty head counts we rely on a Building Manager who has a list from each department within their building who keeps an accruate list of personnel under their control. When someone is off campus (due to illness, vacation, Dr. appointment, etc.) the individual departments within the building are held accountable for their list being accurate. We perform Annual Fire Drills in Administration and Academic Buildings and the lists have been as accurate as they can be due to peoples visiting other buildings and may not be in the building at the time of the evacuation or a drill.
For Residence Halls, the Student Life Department is responsible for keeping an accurate list of Students and Faculty who reside in these buildings. We perform quarterly fire drills in these buildings. As with the other buildings, students don't log out and log in and they may be at an off campus activity or attending a class when an evacuation or drill is conducted. We do the best by work of mouth as to the accuracy of the list.
This is how we conduct drills and verify head count at the Assembly Point designated for the specific building. I hope this doesn't add to the confusion that this subject can creat. It all boils down to what works best your your type of business.