It's great if school officials want to enforce some decency standards, turning a kid away from her prom because of a dress that was previously approved by a school official (in this case, the same one who turned her away) is idiotic. Ms. Gardner should, and probably will, be sued at the very least for the cost of the dresses and prom tickets.
"We can't go back to prom night. Prom's over. Our prom has been stolen from us."
If I was a judge, I'd probably make Ms. Gardner pay for a whole second prom.
It’s entirely possible that the dress may have been approved by the school official, but not the ample body that showed up on prom night hanging out of the dress.