It’s not about the dress. It’s about anal retentive little dictators who are guided by a 12 inch ruler and not by reasoning or feelings for an adolescent who has probably not been in trouble one day in her life.
Your are putting onus on a teenager. It should be on the adults who should know better. But why act like a mature, feeling, reasonable adult when you can be petty and ruin a dance for a nice girl?
They have to be guided by a unit of measure otherwise instead of a simple modest dress that fluctuates by just a few centimeters if at all they are faced with really slut couture. Yes in a sane common sense world a simple “modest attire that does not expose cleavage or is too form fitting” would be enough of a rule to keep clothing attractive yet decent. But that world left the schools a long time ago so in the interest of clarity and fairness they have become way more petty than schools of old.
It really is not their fault as much as it is students and parents who think the rules are always meant for other people. It also has to do with the fact that the dresses available for such events have become way over the top compared to the semi formal dresses once seen at earlier school events.
Bottom line though is that there were alternatives to her having to wear her winter coat. The rule was not that onerous and could have easily been complied with if she had shown some foresight. I would hope what happened would not be considered “getting in trouble” on her school record.