I am in Plano, just a tad north of Dallas in the burbs, and the girls at our high school wear short dresses for homecoming and long dresses for prom.
That said, there is nothing wrong with wearing a dress you love and look great in, short or long!!
Short dresses can be found... it just depends on where you look. Terry Costa has some pretty dresses. We got daughter's prom dress there. Beautiful! We actually found a couple of short dresses by surprise at Dillards.
So I suppose you will be getting the mum as well... That is quite a expense in itself. My daughter had a wonderful time with homecoming and prom.
I hope your daughter has a great time. These last 4 yrs of school fly by.!! My daughter graduated this past June.
ETA: I don't know about other schools but our schools adhered to the school dress code at Homecoming and Prom. Some girls were turned away from both. It is ok to look nice and beautiful but the school does draw a line on the cleavage and butt cheeks. So, if the parents choose not to buy something appropriate, the school will parent for you and not allow the girls in to the event.
ETA2: I complete agree with Meganthemom on the spandex under short dresses. My daughter was in cheer all 4 yrs and captain last year. She still wears proper colored spandex under short dresses. I'm talking about short, boy short type spandex that is worn with cheer uniforms, etc