For some reason I seem to be only recommending crossovers.
Title: The Marlows at St Mildred's
Author: Mrs Redboots
Period: Switzerland, not long after the Triplets have left school
Characters: Nicola and Lawrie Marlow, Len Maynard, Miss Wilson, Jane Carew and mother, Jack Lamber, Margaret Twiss
Why I Think This Story Rocks: I had never come across the Marlows before I read this, and bought Autumn Term as a result. It is well written and sound like Antonia Forrest (based on the one book I have read!)
Title: A Chalet Girl at Last
Period: Switzerland - when Blossom and Sybil etc are at St Mildreds.
Characters: Sybil Russel, Blossom Willoughby, Gwendoline Mary Lacey (from Malory Towers).
Why I Think This Story Rocks: It is well written and enjoyable, and it show the redemption of Gwen very realistically, especially considering she was given rather a raw deal at Malory Towers (who else hates Alicia now they are grown up?).
Title: Veronica at the Chalet School
Period:Switzerland of the Mary-Lou and Vi era.
Characters:Veronica Weston, Sebastian, Caroline and Fiona Scott, Mary-Lou, Vi Lucy.
Why I Think This Story Rocks:I adored the Wells books growing up, and didn't actually come to the Chalet School until I was an adult. This brilliant piece of writing clearly captures Lorna Hill's voice and characters perfectly and I really enjoy reading it.