British boarding school series from late 60's featuring twin sisters?
I'm wondering whether anyone could help me find a British children's book series that was about two identical twin girls and their adventures at a British boarding school. My sister (an American) borrowed the books from a friend who had bought them in England way back in the late 60's when she was a little girl. Since she never owned the books, she's forgotten the author and titles, but she remembers them fondly and it would be fun to buy them for her as a nostalgic present. I remember that each title included the school year: First Form, Second Form, etc.
I once tried doing that same search and came up with the same hit, but if you'll notice, those Malory Towers books are about a single girl named Darrell, not twin sisters. I have a feeling that the books I'm seeking are out of print and existed pre-Internet. I even asked about them at a children's bookstore when I was in England in 1997 and it didn't ring a bell with them.