Surely this already exists and a bit of research would turn up whether it serves this exact need or not or if it needs to be adapted. But how about if a school puts in little scanner thingies on every classroom doorway and then requires each child to wear a badge or something. If a child hasn't come through the door at the specified time a text is instantly sent to the parent. Although this may be handy for all kids (teens will game the system of course) I'm thinking more for primary school-aged kids. This would prevent the dreaded situation of a child being forgotten in the car in the parent's work parking lot or left sleeping on the bus.
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