Instead of agreeing to go home and change or to put something over it (various accounts said he had a trenchcoat with him so he could be a flasher if required to cover up), he ran when confronted. He then put a sheet over his head to look like a ghost, unknown to the administrators as they had a quick huddle to decide where each of them would go to look for him -- while he was standing within earshot in his new disguise.
When he was finally caught, he was given a 3-day suspension with the explanation that it would have been one day alone for the dress code violation, but he got a second day for running away and a third for insubordination.
After we heard about it early Friday afternoon, we had a good laugh. It got better that evening as we were leaving the Halloween event at the high school and found a slip of paper taped to the inside of the front entrance doors with tape obviously ripped off another posted piece -- a closed fist bracketed by the line "Free Besley" (the last name of the 'streaker').
Since the janitorial staff automatically takes down anything posted without the approval stamp/initials from the school office, I took a picture for posterity and snagged the slip to show Mom later. I'll refrain from making any jokes about distributing it since she does have the authority to use said stamp.