Well you made the first assumption, that she knew she was breaking the rules. I contend that she knew no such thing because the dress is so old fashion and modest.
I’m betting my assumptions are closer to the truth than yours.
If the students were given the rules before the dance then you would be wrong. I doubt they were just handed the rules at the door.
The dress is lovely and I think very modest. But this is not about the dress. It is about snowflakes. She could very easily have taken a picture of herself wearing the dress and had it vetted before the dance. She also could have gotten in touch with her mom and asked her to bring a shawl or other covering besides a winter coat. She could have nicely asked for the straps to be measured. But no she wanted to play the poor put upon act and call the news.