You're pretty sure, but you don't know for certain. Based on that article we have no idea if she was asked to leave because she was dressed informally or because she had pants on.<quoted text>
What do you mean formal wear for men? Women at my work cannot wear a man's suit. They can wear a woman's business suit, but they can't wear formal wear for men. I'm pretty sure something like that (a woman's business suit) would not have resulted in her being removed from the prom.
If you mean what if she wore a man's tux, I feel if a girl wants to make that kind of a statement ... that's fine, but she should do that on her own time or organize her own formal function where women can show up in tuxes. I wouldn't have a problem if the school wouldn't allow a girl in wearing a man's tux, just as I wouldn't have a problem with them not letting in a boy dressed in a prom dress.
Being the literal pain that I am, the description of acceptable attire was formal, it didn't state she had to wear gender specific clothing, if she had a tux on she would have been in the right.