We use Meili GPS to add a layer of security to our DofE expeditions. It's important to give the children as much freedom as possible and since like most other schools we don't allow phones we use these to make sure we know where each group is at all times. The SOS button works really well and they work everywhere we have tested them so far. Rural Oxfordshire, north Wales, The Brecons , the New Forest and the lakes.
Even when there is no emergency it's really useful to be able to see how groups are progressing around their chosen route so we can be sure to have staff on hand at checkpoints at the right time. And when groups get lost we can go and intercept them and post them back in the right direction.
The battery lasts at least a week and we have them set at 3 min updates. Not sure how long in total as they have never run out but I would guess a couple of weeks.
Five stars.