I feel sorry for my pupils when their parents are hell-bent on aiding them in breaking the rules. During my year seven lesson this morning a ringtone blasted out playing the Saturdays. The poor girl whose phone it was was in two minds whether to turn it off as soon as possible or try to keep her head down and hope it'd go unnoticed. The latter was never likely as the class was told repeatedly that the group 'just can't get enough'.
'It's my mum' she says and sure enough on the phone as she takes it out of her bag it says 'mum calling'. 'She keeps thinking lunchtime starts at twelve o' clock.' I took the phone off her until the end of the lesson but didn't send it up to the office for keeping until the end of the day. I'm not sure what hope my pupils have when their parents go about sabotaging maths lessons.
menhir

Pardon me, but there should be an unbreakable rule that phones are switched off during class periods. Your pupil can switch on when lunch time arrives and pick up missed calls etc from mum then, at the right time. Parents and kids will learn a little structure from this requirement, one that should be backed up by strict school rules. These rules, parents should be told, that way there is absolutely no excuse for misunderstanding, except for language barriers that may need overcoming.
If phones are not switched off then they should be banned from being brought into the class room, or the school (though I can see out of school safety arguments rearing up here).