I'm somewhere in the middle on this. The school has the right to confiscate the cell phone. And, I truly believe that if it was confiscated because it was being used, the school has the right to see what the recent activity was on the cell phone. I don't think they should be able to dig any deeper than that.
However, school administrators have the right to search through a book bag if there's suspicion that a student is breaking a school rule. I wonder if the courts will actually extend that to electronic searches in a cell phone? Clearly, the student was breaking a school rule, thus one part of this argument has already been satisfied.
The schools I'm familiar with don't operate in vacuums. They're very aware of current case law. We receive dozens of cases for the teachers to read over to help us see where the courts currently stand on issues. I believe lawyers were involved when our district crafted its code of conduct. One of the biggest cases lately was in NYC when they had a complete ban on cell phones in the school. The district lost the lawsuit, but within the opinions in that lawsuit was enough information for districts to revise or strengthen their own rules. For instance, in ours, if your cell phone is taken, your PARENT must pick it up at the end of the school day, not you. That's because the parent is told in person that if the student is using the cell phone again during class, it will be confiscated until the end of the school year. Some of you are reading this thinking "they can't do that." Oh yes they can & seem to be on very solid legal ground to do so. The reason the parent has to come in to pick up the phone is part of due process.