I think that a text messages from parents about changes in plans that students receive upon taking phones out of airplane mode at the end of class eliminate an real drain on school secretarial resources. One message every few days to your kid may not seem like much, but school-wide it amounts to dozens and dozens of such messages a week, each of which takes at least a couple of minutes to pass along, amounting to hours a week.
Absolutely, if I found that I needed to communicate changes in plans to my child every FEW DAYS, a cell phone would make sense! But in the five and a half years he's attended public school so far, we've had to do this exactly once--to tell them that I was in labor so his godmother would be picking him up that afternoon. The rest of the time, we make our plans in advance and stick to them, or if something is ambiguous we have a backup plan: "If Daddy's not home yet when you get here, go to Abby's house."
If I lived in a remote area and/or a place with unreliable utilities, like some of you, a cell phone would make more sense for me as well as for my child. As it is, our everyday routine keeps us always within screaming distance of other human beings, and I know from experiences fainting on the sidewalk that Pittsburghers love to rush over and make sure you're okay! We have never had a power outage of more than a few minutes in 13 years living in our home, and in all the places I've lived in 25 years here, the only landline outage I've had was when someone apparently cut the wire to my apartment, which was creepy but not the kind of thing that happens often, thank goodness--and my downstairs neighbors let me use their phone. Even when I travel alone, usually I'm either on public transport or driving on well-traveled highways where I could easily get help if needed.
As for the kid living on her own while she goes to high school, I certainly understand why her having a cell phone makes it easier for you to feel comfortable about her safety and your ability to communicate with her, and I likely would want my child to have one in that situation, too--but I still don't see it as truly necessary. I went to college 1,400 miles away from my parents when I was 18. I had a landline in my dorm room, with no answering machine. If they couldn't reach me, they had to assume I was out somewhere, oh well. I called them at least once a week, so not all that much time could pass without any contact. And then in my second year, my dad got email at work and I already had email at college, so he could leave me a message that way and I'd answer as soon as I saw it, typically within a few hours. Yes, I was a legal adult, but I know my parents did worry about me a bit--but based on what I hear from parents whose cell-phoned kids are away at college now, the level of worry is about the same anyway.