I understand the sentiment and would like to believe that this letter has good intentions behind it, however misguided I believe it to be. I am by no means an expert in law, but an expectation of privacy in public in this day and age is non existent in a high school gym. He was not texting in a bathroom stall of a public restroom, or a designated area that would provide him with any expectation of privacy.  He was sitting in the bleachers in a high school gym, I would expect that a person with his title would understand what that means in regards to their public behavior. 

I also agree with the post of me>you above regarding handling the bullying topic privately. I do not believe that taking bullying public has made the issue "infinitely worse" as you have claimed. To the contrary, these girls now know they are not alone. Fellow students have staged a walkout today to protest the Principal's behavior. I would imagine  the victims appreciate the support by their peers.