Ah, but the concept of public is a misnomer. The public schools are actually not just open to the public, they are in fact government entities. Try entering one without a justifiable reason to be present on campus. That is where the rub is here. They cannot, by court law and constitutional interpretation, allow anything resembling prosletizing as a part of an official school function. To do so, particularly without allowing any and ALL other faiths to do so is in fact taking a stance on one religious belief over others.
One point missed here is that the valedictoruian had this explained when her draft speech was given back to her and she was directed to remove references to religion. She was not unaware of the issue, she chose to ignore direction given to her by the entity responsible for the school and the school's functions. Make no mistake about it, a graduation is not an open "public forum", it is a school sponsored, directed and official activity.
I may not like the fact that she had her microphone cut off, but she was not at her forum, but the school's forum. As she was entering in the "adult world" she just got a lesson that you can't get your way all the time especially on someone elses dime.