From Day One, teach your kids to line up in a particular order. You can do this by their first name, last name, or by any other criteria. The students should learn to know who is in front of them and who is behind them, as well as who is at the front and at the end of the line. This will help you when it comes to fire drills or other emergency procedures where you must know if someone is missing quickly. The students will most likely know who is missing before you do. You should simply be able to ask everyone to make sure their neighbors are there.