The school trip
Counted at every door. Nobody left on the bus.
A trip is a head count done four times by a tired adult: onto the bus, off at the zoo, back on, off at school. Each count is a number, and a number does not say who is missing. The handheld counts by name.
A wave makes the list
Nobody prepares a trip list. The teacher picks where from and where to, and each child waves a hand or taps a card in passing. Whoever boarded is the list, by name and face.
The same list at every door
Off at the zoo, back on, off at school: the same children, counted against the same list. Whoever has not waved yet stays on the screen by name and photograph, not as "one short".
The office sees the bus
A child on a trip is Off site in the register, not absent. If the alarm sounds at school while 2B is at the zoo, the roll call does not look for them.
It is the register, not an app
The handheld is the one from the gate, and the trip is a sealed record like any other: which places, when, who ran it, every child with the minute. It works with no signal at the zoo.