Common sense guidelines where I'm at.
If a student has a mobile out in the lesson, I give one warning. 'Switch it off & put it away'.
If I see another mobile that lesson (ie. I've told student A to put his away, student B still thinks it's OK to be fiddling with hers = student B is taking the piss), then I confiscate.
Perfect behaviour for the remainder of the lesson means they get it back at the end. If not, I do the 'school policy' thing - it's passed to the front office, & a form letter's sent out informing parents that their dc's phone has been confiscated, & they need to come in & collect it.
I disagree with the school policy, personally, because I think that mobiles have massive educational implications & we should harness & manage them, not obstruct them.
For example I stick an image on my IWB most days & challenge every class I teach to 'find the reference & tell me something about it' - like this for April
wasteland lolcat
It was one of my lower abilty year 8s who was savvy enough to search 'april lilacs lolcat', generating a brief discussion of TS Eliot's personification of 'April'...
So basically, OP, the answer would be that most schools officially ban them, can't effectively enforce that, & miss opportunities to use them. Best policy for a student is usually to keep it on silent & in the bottom of a bag.