I'm of the mind that the school had no right to ban her. She didn't work for the school, wasn't obstructing anyone, and regardless of what the FFRF say, it is not unconstitutional. Giving the impression that the school supports a particular message is a far cry from actually officially supporting it. Banning someone because they might offend someone's delicate sensibilities is wrong.
The school administration are chickens, and took the easy way out bowing to pressure than stand up for what is right. Suppressing the freedom of speech and expression is absolutely unconstitutional, while protecting someone from what they may perceive as the school sanctioning a religion is quite a stretch.
Sourced from misguidedchildren.com: