This school has a high percentage of upper-middle class and “1%er” families. It’s really no surprise she thinks she’s above the rules, especially since everyone absolutely has to see her amazing pink dress she bought in Paris.
If she is Mormon, she wouldn’t be able to wear this dress to a church dance, so that’s probably why she was wearing it to a school dance.
She could have very easily taken a picture of herself in the dress and had it vetted by school administration to see if it would be o.k.