Our school uniform was a baggy, sleeveless outer garment with a yoke neck, and box pleats. There was a small breast pocket with the school crest on it.
It was worn over a white blouse, belted, with knee socks and loafers or oxfords. In the U.S., the unlovely garment we wore is called a jumper.
I know English girls wore them too. The Belles of St. Trinnians dressed just like us, except they wore ties. (And our headmistress didn't look at all like Alistair Simms.) But I don't know what they were called, because in England a jumper is a sweater, not a dress.

I found a uniform from our old school at a rummage sale. I sent it to my sister for her birthday. I'm looking forward to her response.