In an emergency situation I would much rather count on a computer to do head counts than a human, but to counter ID's being left behind you would confirm the computer's count with a teacher's. For ease of use, make the system accessible through the internet and a secure login so it can be accessed with a smartphone. A quick check would verify that little Billy was out sick today and not still in the building.
I can speak to this from direct personal experience. My job is automated minibars in hotels. When a guest takes something out of the minibar , a sensor automatically records the sale. In our minibars, the computer really DOES track everything.

Does this mean we have total, complete confidence in the computer? Unfortunately the answer is "no". The computer has a limited ability to recognize when the user picked up a product , changed his mind and put it back. OR moved something from one shelf to another out of absent-mindedness. Or decided to use the minibar as a personal refrigerator, putting his items on our shelves.

The mis-use cases are endless.

Which is why we have hotel attendants whose job it is to visit every minibar with a mobile device and reconcile the records of what the attendants see with their eyes with what the computer records say. Well, that and the bars can't restock themselves ... yet :). At any rate, false sales (sales recorded when they shouldn't be) do occasionally occur and so do attendant sales (sales NOT recorded when they should be, so the attendant manually does so). I suspect the exact number is company proprietary and specific to the hotel, but it's enough that giving the attendant reconciliation features is a good idea.

If the system used to track students is anything like that used to track products in minibars, I would NOT rely on the automated system in an emergency. Humans are much more unpredictable and movable than products which do nothing but sit on a shelf. I would expect an RFID surveillance system to give a general "rule of thumb" view of the school at any time, but I'd still insist and demand humans in an emergency to catch the oversights in the system.

Respectfully,

Brian P.