We know emergency situations can (and will at some point) happen in your class. It may be minor, such as a student becoming sick in your room, or even a practice event like a fire drill or tornado drill. Hopefully you won't encounter a real life-threatening emergency. But you should always be prepared for such instances.
Fire drills are probably the most common situations you will encounter. The best way to handle these is to teach your students what to do in the event of a drill or an actual evacuation. Yes, you can teach this to your students. Fire drills are to be surprises only WHEN they occur, not a surprise in WHAT to do. It is good practice for your students to know exactly what the procedure to follow is. The most important part is to be sure YOU fully understand the school's fire drill procedure and you can confidently teach it to your students.
Making sure all of your students are accounted for is your main responsibility. Thus, your attendance taking is very important. You want to make sure you have a means of carefully checking attendance when you and your students reach your destination. Have your grade book, attendance sheets, or a class roster easily accessible and always in the same location so you can grab it as you leave the room. I use the class roster file on my handheld because it's always with me.
Teach your students to exit the room carefully yet quickly. Instruct them in which direction to turn from your doorway, and what exit is to be used. Always have your kids line up and stay organized so you can take attendance easily.
And let them know why it's important to maintain composure and control, not playing or wandering around. If you are new to the building, your students will probably already know where to go! The trick will be getting them there quickly and maintaining order.
You'll want to let the students know how to react to different situations. They may find themselves in the hallway heading back from the library, in the rest room, or involved in a group activity in a far corner of your classroom.
Obviously more urgent matters will constitute true emergencies, and it is very difficult to prepare for these. Hopefully your school has a comprehensive plan to cover bomb threats, intruders, inclement weather, and other emergencies. Take time to carefully read through and understand these procedures, so when an emergency does occur, you can confidently lead your students. The students will respond to you when you give direct, confident directions.