I agree that we should leave religion out of the public school, but I don't think people should make a big deal about a banner here and there, unless it starts getting overboard.
Like when it came to the Terms of Service here. People weren't supposed to speak sexually or use profanity. A curse word here and there or a innuendo didn't bother anyone, but when it broke out all over the forum then people complained. I can understand opposing it when it goes overboard, but not a little word here or a little banner there.
When I went to Jefferson Highschool there was a paper on the door of a classroom that said, "For the wages of sin is death, but the gift of God is eternal life through Jesus Christ." Romans 6:23. I don't really see that as a big deal, even though I'm not a christian now, as that can have various meanings. The Hebrew word for sin means to miss the mark or to be out of the way. Based on that one may interpret that verse as saying, "The result of being out of balance with the pure nature, which is the result of ignorance, is suffering and the death of peace, but the gift of God is the teachings of Jesus Christ, which lead a man back into union with the pure nature, if one abides in this example; such as serving others, being as children, and not worrying about things." But if one was telling others, in Jefferson Highschool, that they'd burn forever for not accepting extraordianry claims without evidence, such as blood sacrifice needed for god not to be angry with you anymore, then I'd have a problem with it and say it's went overboard. Such ideas can cause distress within people, and that's not needed in public schools.
Christianity can be interpeted as a very reasonable philosophy if interpeted as a metaphor, and I have no problem with one quoting Jesus, anymore than I do with one quoting Socrates or Nietzsche. But hey, Jessica may be offended if people pray for peace in her city, so remove the banner...