IMHO the catch is that older people (teachers) translate their experience to younger (students). Teenagers are just naturally better at multitasking because they are yet to be affected by brain ageing. And school personnel can't imagine that someone can be using phone and listening at the same time. Now factor in that most 30+ crowd didn't grow up with smartphones, and teenagers are just used to doing stuff while being on phone from early age and we have an answer.
Great point and I think it is some kind of adaption or evolution, if you will.