I agree with Dave, the policy was probably written to address something else and was written vaguely so there wouldn't be any loopholes. Unfortunately the Law of Unintended Consequences reared its ugly head.

I also concur with the pre-prom discussion idea. Have a meeting with the principal, show him/her some photos of outfits similar to what your son would be wearing, and ask if that would be acceptable. If the answer is no, pull out the big guns (not literally). Show this photo of HRH The Duke of Rothesay (more commonly known as Prince Charles) and ask if he would be turned away because of his attire: