...it was not a freedom of speech issue, but a concern over the depiction of violence.

So the right to depict violence in art is not a freedom of speech issue?

I can see the school administration getting alarmed if the violence depicted was aimed at another student or a teacher. That could definitely be interpreted as a threat. But George W. Bush's head on a pike is not a threat. It's 100% symbolism and anyone that doesn't realize that has his own head up his ass.