this is the only part of your post that i was addressing. people can wear what they want, but HS students are not constitutionally entitled to wear what they want to a HS dance.
I don't necessarily disagree with you here, but I feel this should have been a SHORT SKIRT issue, not a thong issue. How would wearing full-on granny panties and a short skirt have been much different? You can avoid the whole underware issue when the school sets limits on the shortness of the skirt. Like an inch or two above the knees? I'm not sure exactly was is appropriate because I've never worn a skirt (and my buddy Byron is a liar if he tells you different - a kilt is NOT a skirt!). There are other acceptable clothes the girls can wear to a dance - thong or no thong.