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Make sure to leave the sidewalks clear for the firefighters to do their work. If there are too many people clumped on the sidewalks, firefighters cannot get through. Be sure to listen for people in authority giving directions. Teachers or your bosses will likely be looking to take a head count, so they will want to get everyone in the same area, which is why it's important to stay quiet. If there really is a fire, the building could eventually collapse. You should move a safe distance away from the building. Generally, across the street is fine. Don't assume that because the fire alarm has stopped, you can re-enter the building. Wait until the firefighters or someone else in charge tells you it's fine to go back inside. Once you hear that, you can resume normal activities.
Clear the sidewalks. Move to a safe distance. Wait for the all-clear.