It seems to me that everyone else who has said anything in this thread is failing to consider the true issues involved.
Consider the fact that most of these schoolchildren are expected, eventually, to grow up to be adults, and to try to function as productive members of society, in many instances by holding and working at jobs.
They need to learn to balance their use of cell phones, and the demands that cell phones tend to make on one's attention, with the needs of their jobs and other aspects of living in The Real World. From what I have seen, to many adults have not learned to manage this balance, and nothing has been suggested in this thread that will help anyone to learn it. Only the brutal misuse of force by tyrants, to rob people of their rightful property has been suggested a a solution, but in the end, this is no solution at all.
And I have seen the fruits of what this will unavoidably produce. I've had coworkers who allowed their cell phones to deeply cut into their productivity at the job; coworkers who have been conditions to see their supervisors as tyrants, merely for demanding that these workers do their jobs rather than giving all their attention to their cell phones. I've seen twelve-thousand-pound forklifts zooming down warehouse aisles, carrying up to their six-thousand-pound capacity of cargo (eighteen-thousand pounds total GVWR), being driven by idiots who are paying too much attention to their cell phones, and not nearly enough attention to where they are driving.
I've seen nothing in this thread that suggests any genuine solution to the genuine problem.
The cell phone genie is out of the bottle, and is not going to be forced back into it. People are going to have cell phones, whether they are students in school, or workers on a job.
We need to be teaching people, from an early age, how to maintain a proper balance between cell phone use, and allowing their attention to be sufficiently on other thins where it is required. A modern cell phone is an incredible tool, that can be used for great purposes that can increase one's capability and productivity; but it is a two-edged sword, that can also have devastating impacts on capability and productivity as a result of its capacity to distract its user's attention from where that attention needs to be.
One major capability of a modern cell phones is the ability to quickly obtain information that might otherwise not be immediately available. Surely, anyone can see how this ability, properly applied, could be a great asset in an educational environment.
It will take someone with different knowledge than what I have to figure out the details, but I think it is clear that there needs to be a different approach in education, than what anyone else has suggested, to dealing with students' use of cell phones, and teaching students to make good use of this resource, and to avoid allowing it to distract their attention away from where it needs to be at any given time.
But it probably won't happen. It's much easier for teachers to become tyrants, bullies, and robbers, than it is for them to address this matter correctly.