Here is an article from the London Times with a top 25 boarding school stories - most of which I agree with (a few I have not read!):
5) The Autumn Term, Cricket Term and Attic Term by Antonia ForestForest wrote only 13 novels, all but one connected with the Marlow family. Ten of them were about the Marlow children, beginning with the Autumn Term (written in 1948) and were fabulous tales of the six girls and two boys. They deserve to be better-known. I particularly remember Nicola and Lawrie, the two youngest sisters, who didn't quite live up to the standards of their hugely successful older sisters at Kingscote, the family boarding school.Autumn Term, and its sequels, The Cricket Term and the Attic Term and were funny, beautifully written, and seemed far more realistic than many of the other boarding schools books around (especially when it came to the relationship between all the sisters).