I was a bit slow off the mark with this one - the first step is letting someone know about it. In this case, the school administration. Their email address can be discovered by doing a quick bit of Googling, and in the past I've found that if you let the school know about these things, they are taken care of pretty quickly.
Like I said, I was slow off the mark here, and this graffiti hung around for weeks. I'm not altogether sure why none of the teachers noticed it (probably because they all drive to school rather than walk, and therefore go past the outside of the school too quickly to notice any problems), and I also don't know why a parent didn't notice it and report it.
I guess the answer is the same as always - many probably did notice it, but they all decided it was somebody else's problem.