[BigV]

From here.

...Kevin Logan, who attended school all year wearing female clothing, was denied entrance to the prom because he was in a dress.

The school uniform dress policy, which quotes the First Amendment right of freedom of expression, prohibits halter tops, miniskirts and attire promoting profanity, among other things. It does not say boys cannot wear dresses or skirts.

However, a clause in the code says administrators, at their discretion, can ban clothing they think would be disruptive.

That discretionary judgment is what concerns Badgett and her girlfriend, Laniqua Gaines, who said it’s a window for prejudice that makes victims of boys such as Logan, 18, who prefers to present himself as a female.

Emphasis mine.

I don't see where this says it was the first time he wore a dress. Based on what I have read so far, this is a case of the administrator in charge at the door making a judgement using their discretion. I think the decision is a very bad one, and represents a losing cause in court, should it get that far. So what is the story? He was banned because he was wearing a dress? Or he was banned because his attire was disruptive? Or because his dress was disruptive? It comes down to the administrator's judgement. Perhaps they have a legal leg to stand on; it's a pity they're using it to stomp on this kid's prom.

Also

"Ms. Rouse said I wasn't allowed to have on a dress," said Logan, who noted he wore female garb the entire school year.

In a 1999 Indianapolis case, a court ruled males can wear dresses to high school proms as a First Amendment right, the ICLU's Ken Falk told the newspaper.