The school has an emergency plan and you usually get this before the opening of school for students. Most experienced teachers store these away and will pull it out when needed. I suggest that you check out the plan as if you had a classroom of students and make your own classroom emergency plan. Decide where you will keep your roll book for easy access in case you need to leave the room quickly. Go to the appointed place so you will know where this location is and the route you need to take.
Once you have your students, this emergency plan should be one of the early things you do. I grouped my students into groups of 4 and made one person a leader. When we got to the appointed place, I asked my leaders who was missing in their group. This was for immediate attendance and I could go back afterwards and eyeball that each student was there. I would take the students on a practice drill to the appointed place so they could learn the route and the location. At this time I would also share my expectations with them on how we would get to this location. You might want them to stay in a straight line with no talking or stay in their groups. However you want to work this, now is the time to deal with this and not during an official drill or real emergency. You might also assign someone for turning off the lights and closing the classroom door. If you on the high school level, you will have more than one class that you will need to do this for. We also had a different kind of plan if the emergency happened during lunch time so that needs to be discussed also. After physically practicing the drill at the beginning of the year, I would discuss the procedures at least once a month and of course our school had an actual drill once a month.
I did this for fire and tornado drills. By doing this beforehand, I was able to have a well organized and disciplined class during official drills. Students, other teachers, and the administration will notice that you have control and during emergencies this is very important. Many teachers take emergency plans too lightly and as I see more violence in the schools, I think we need to take this seriously and the more we take it seriously, so will the students.
Photo credit: Fire Truck by Happy Dave