The first "dress" photo showed the daughter and mother. The other dress photos showed the daughter and another girl, perhaps another daughter. The group photo shows the mother on the left end of the group and the two girls in their prom dresses in the middle. There wasn't a dress switcheroo.
Prom shots is about the person, not the roset, the landrover, the fence or any other background crap. Let's see the subject pop out from the background.
Try 100mm small apature, shallow DOF, fast shutter, good soft lighting. No background crap