As a retired New York City school teacher, what I can tell you about this topic is that cell phones should not be allowed into the classroom. Any student that enters the classroom with his or her cell phone simply shows lack of interest in education. Such a student can't pay attention in class, and will have more time for social networks.
When I worked as a public school teacher in NYC, I often caught student texting or playing games with their phones whenever I turned from writing on the blackboard.
By the end of every term, most of the students wouldn't be able to pass the exams, with several of them dropping out of school completely. This high rate of failures among students should be blamed on certain classroom distractions like the use of cell phones.
To most parents, cell phones are easy ways to get in touch with their kids during emergencies. But I often wonder the kind of emergency that will warrant calling a child on his or her cell phone during school hours. I would think that getting in touch with either the police or the hospital would have been more appropriate. And such parents can always reach their children through the phones lines in the school's main office.
And it is a joke to suggest that students can keep their phones locked away in their lockers. Once they have cell phones, nothing stops these kids from bringing their phones to class.