You make all kinds of assumptions about the school board being "sympathetic" towards her, and being prepared to allow her to attend with her gf as long as she goes along with the charade and keeps her mouth shut, without any evidence (that I can see) to back those assumptions up. If you really think all those things, no wonder you think this girl is a selfish bitch who ruined it for everyone by turning things political (though even under those circumstances I'd have to say her actions would be quite understandable). But if you don't make those assumptions, and take things like what Macharius said about the past treatment of gay couples into account, then suddenly the picture looks entirely different - wanting to take one's significant other to the prom and dance with them isn't exactly what you'd call bringing the politics in.

So really, you're going to have to make some stronger arguments about why you find your assumptions justified. After all, the school went out of its way to explicitly state couples had to be one boy and one girl - that doesn't really sound like they were that sympathetic to her.