Dance expert, Haley Moorse, teaches a couple of drills to help improve upper body posture while kicking.
One drill to work on proper upper body technique is to have dancers connect at elbows, rather than shoulders. This makes dancers really focus on their upper bodies since there is a lot less stability from the elbows compared to the shoulders.
You could also have dancers face every other front and back and kick. This will allow the dancers to feel if they are hunching and/or leaning to the side.