When children are at a certain age where they can go off out without an adult then I would say a mobile phone is an excellent safety back up.
Younger children at primary and junior school are usually taken and picked up by an adult ..................
Older children at senior school, I would think almost all of them have a mobile ( if they want one )
As far as I know pupils at senior schools all have lockers therefore it should be mandatory that all pupils put their phones in their lockers at the start of school and retrieved at the end of school. There should be no reason for the pupil to need their phone during lesson time. Pupils not abiding by the school rules should have the phone confiscated until the end of the school day and maybe get a detention. Older children can get into silly and sometimes serious predicaments so yes I would say ..... these days why shouldn't they have one, even if it's only for peace of mind.