On the whole issue of cell phones versus no cell phone allowed in class, I think there are situations where a student should be reachable by others. So simply banning cell phones or keeping them in the teacher's desk may not be acceptable. Sure a classroom is a special place where all the students and the teacher need to communicate together, but let's face it: It is not the single most important thing in the world. Keeping students incommunicado for hours or whole school days is very like kidnapping.
Maybe we should recognize this fact and let students physically excuse themselves from class to answer their cellphones. Maybe we need something like a parental override for when a cell phone is switched to "in class" mode. An override could cost $10, so it would not be abused. Switching phones to "in class" mode would be done automatically by a little radio in each classroom.
Apparently this girl was in some kind of situation with foster care and the death of a family member in which talking on her cell phone (just maybe) was more important than classroom participation. If her mental state was so fragile she considered her line of communication more important than her education, then the school should have offered her a quiet room where she could talk. Certainly not something you do everyday for all students, but it should be possible to recognize and respond to special student needs.
As adults with cell phones we all make this judgement for ourselves: Most of the time we shut down our cell phones for important meetings, but under some circumstances we don't and will excuse ourselves to go answer them.
