Kids should be allowed to have cell phones at school. What if a parent needs to contact their kid for some reason or a family emergency? Schools do not always make it easy to pass info along hence why the phone is needed. Yes they should be off during class, but allowed at school.
Children and teenagers will find ways to distract themselves in school - this is a fact that is as old as school in general. Cell phones are simply newer ways for kids to distract themselves; while they're likely to distract themselves anyway, there's no reason that they should be allowed to have the chance for further distraction.
Cell phones, per se, do not cause concentration problems. This is a matter of personal responsibility for the kid. If the sounds bother them, they can simply turn the volume down or turn the phone off. The ability of the child to concentrate has nothing to do with what's around him/her. In fact, having one around can actually be a good way for the child to learn how to concentrate.