Yes, entry and exit are not easy to track reliably. Instead of placing these 'short range cell phone identifiers' at entry points, we could distribute many of them around the premises.
Or we could just use a single receiver capable to measure the power level of the signal to determine if the phone is 'near enough'. If this could be made to work in practice (it might be difficult to 'tune') then we'd know at all times which phones are to be punished.

But I will not insist, I realize this idea has a lot of loose ends.