Be sure not to sue the school district

because it’s not the school district’s fault you got booted from your prom.
No, the correct avenue would be to sue the store that sold you the revealing dress in the first place.
It’s a prom, damnit, not a runway show or a pole dance. Have some respect for yourself. Cover those things up. And the dress shops should understand that, which is why in my opinion they ought to be sued.