Having attended school and worked in one for 12 years, I'm not aware of any incidents where a student had to call 999 in an emergency.. I know a single anecdote isn't evidence, but I think the incidence of what you are suggesting ever happening is vanishingly small.
Even in a huge secondary school, I can't think of anywhere a pupil would be that isn't within shouting distance of a teacher or adult member of staff, or a desk phone during school hours. There are adults all over the place, classrooms, corridors, kitchens, halls, gyms, playgrounds, fields, libraries, offices. Students are not left to their own devices unsupervised at any time.
We don't have the pouches yet but do have a no phones in school policy, and it's only a good thing.
Sorry, edited to add, I missed the part about your child having an allergy which might require the calling of 999. I can see why you would be more nervous about not having instant access to a phone. But I still think it would be very very rare for a student to have to go and find an adult if something were to happen. At most they might have to shout across a field or run 10 metres to the nearest corridor or classroom. At almost all times they will be in sight of someone with access to a phone and a shout or wave would get their attention pretty quick smart. There will be risk assessments and policies on how to handle the situation, perhaps ask your school to see what they have put in place to mitigate the risk.