There's a Rochas white silk pleated dress in white on the internet that's a bit how I imagine it.
Although probably a bit more covered up and the 'high yoke' indicating an Empire Line like bodice. I think it sounds fairly typical for a expensive seventies designer dress. Although when I said that traditional boarding schools will typically require relatively smart evening clothes, I didn't mean that Miranda's silk dress wouldn't seem over the top and too expensive. Laura Ashley or Monsoon etc, mostly not designer.
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