Actually the ACLU protects the Bill of Rights, which for non-americans are the first ten amendments of the constituion. in theory the group is a good idea, however like most good idea it has been spoiled by bureacrecy. A point once made is the freedom of speech isn't there to protect the speech you like, it's to protect the speech you loathe and hate. The idea being one day your speech may be the one hated.

back on topic I think it is silly, the only thing I can of with a shorts association is they both show legs, and since showing leg for men is taken usually as casual dress they weren't thinking. I just hope someone, anyone gets at least an apology for this kids for happened to him and his date. I know on Utilikilts they had a story about the same thing and in fact an attorney who is kilted wrote the school and copied the letter to several congressmen, Trent Lott is the only one I can think of, who wear kilts on national tartan day.
http://www.utilikilts.com/prom.htm