It is if you’re Linda Fiorentino.
This joke brought to you from the year 1996.
The trouble is that female formal wear is just that - female. A pantsuit that looks like a dress is basically a dress. Real pantsuits look too casual or plain.
If she wants to wear formal wear but not a dress, by all means wear a serious, very cool tux. If necessary, give no warning and have a few friends with their discreet camera phones strategically placed to video any attempt to bar her entry. It would make a delightful Youtube memory for the whole world. She may not win the battle, but making some fuddy uncomfortable on camera would be worth the effort, and she may go some way toward winning the war for the next person.
Who knows? Maybe they don’t really care as long as she doesn’t wear jeans… What are they going to do - have the police haul her out of there? That would look even better on Youtube.
BTW - school is a government organization, a formal event put on by said government organization must meet the same rules that apply to the school itself.
My older brother and I were junior-high-school age when his coach told the class to strip to the waist. Somebody asked, “From which end?” 
Off the thread and 2 days old, but I can’t let this go knowing you are a WNY native of a certain age as I am. We seemed to have not been at the same prom. While my date was lovely in (and out of her dress that night) I would have enjoyed seeing her in your choice of attire turning heads.
Back on topic of the promenade, I say to the OP, urge your friend by all means to go. Fighting all the way if she must, but go. Fight the good fight, flaunt the rules to the edge of insubordination, but go. All late adolesencents should experience the joys and sorrow and laughter and tears of the prom. When else in your life can all that happen in a cycle about every 30 minutes all evening long?
Oh -
and, since so many (especially the young) seem to have forgotten:
A tuxedo:
- Is NOT a “tux”
- Comes in exactly one color: BLACK
- Is worn with formal, and only, formal shirts: Ruffle front, French cuffs.
I’ll leave it to a woman (either by biology or psychology) to explain the rules for the basic “little black dress”
That was the rule at my school (early to mid 90s)
Nonsense; they are kids. Go for the Bavarian Print: http://www.mytuxedodirect.com/mens/formalwear-suits/tuxedos/?mkwid=sKB3eFtG7&pcrid=24505499300&pkw=tuxedo&pmt=e&gclid=CIv3-c2H1rsCFUsV7Aod7h8APg
One of my classmates wore something like this.
The proper dress can go from basic work wardrobe to out dancing wardrobe with a change of makeup, accessories and shoes.
This would be a decent dress, though I prefer midcalf length, works in a chair better.
day shoes, evening shoes.
day makeup, evening makeup.
day jewelry, evening jewelry
day purse, evening purse.
Not unless you want to be mistaken for Don Cherry.
Notice even these heathens to not call their abominations “tuxedos”; they call them “2-piece suits”. Still a stretch, especially for orange.
Just what skin color do you need before you can look even credible in ORANGE?!? The Purple People Eater invasion of 1958 eliminated all of the purple people, and greens never managed to reproduce.
Gawd, I’m glad the rules for males are so much simpler.
Aren’t you folks evar going to dump the heels? They have been causing lower back pain for generations, and still, you think them required. Sad.
My older brother went to his mid-70’s prom in a tux that matched his date’s gown: mint green. Yeah, it looked about as bad as you think.
Please: There are things we simply do not admit. Such abominable taste is real close to the top.