Can a high school principal go through a student's phone?

Can a high school principal legally go through a student’s phone even if the student does not have it at shcool? Child was called to the office. Principal said there is a rumor that he had inappropriat pictures of another student on his phone. He wanted to see his phone so he could look through it. Student said it was at home. Principal called parent and wanted her to bring his phone to school so he could go through it. She told him it was none of his business what was in the phone since he never takes it to school.