Linda Rosen and school dance

I like this film. I enjoyed the book as well as the film adaptation. My question is When Linda Rosen comes to school with a black eye; Sandy Dennis (Miss Barrett) asks "Is there anything I can do?"
Linda Rosen replies "You can get us a school dance. Every school has one"
In the background of that scene, on the walls, are murals advertising a school dance.

Slip up? Did actual student assembly have dance getting ready, and producers,etc didn't see posters and murals?
Mike