I don't see this being a big issue. There's a difference between having permission to paint the wall and going out and spraying a random bridge for example. Think of it this way, how is there a difference between spraying grafitti on the wall or painting a mural on it? (with permission
)
The high school 237 is based out of used to have paintings that students did in art class on the walls. The paint was everything from what you could call modern art to graffiti to picture of the Statue of Liberty. Most of the walls of the school were painted when I went there as a student. After they repainted the complete school in the late 90's most of these paintings were painted over. The school today is basically a two tune green paint scheme, not as interesting as it had been.