Short prom dress?

[isaidso]

dd is wanting to wear a short dress to prom, says she won’t be the only one even though both best friends are looking at long dresses.

Are short dresses at prom a thing now or is it still supposed to be full length dresses?

It's whatever your DD is comfortable wearing. If she's more comfortable wearing a short dress then it doesn't really matter what everyone else was wearing.
There was a real mix at my DD's prom- girls in long and short dresses and jumpsuits