Ms. Duncan spoke to the press, and admitted that she initially hadn't been comfortable with the length of Clare's dress. As she pointed out, though, she did eventually let her in. The secondary problem from Ms. Duncan's point of view was the fabric. It was spandex, and so it rode up.
If Ms. Duncan is to be believed, by the time Clare reached the dancefloor, the dress barely covered her butt. The volunteer also defended the chaperones, insisting they were only there to prevent misbehavior from the students.