So long as they're not smoothing their skirts, they can be dressed however they like. Entirely too much skirt smoothing going on.
I don't see it as sexual either. So far as the punishments go, it's not just that bare bottoms smack better; shame is far more punishing than a beating.
In all honesty, if any movies are ever made, I hope they tone down on it some. Seems it might be a bit too distracting to have Berelain nearly spilling her cash & prizes every time she bows.