I wrote a post last year about my experiences with sapphic boarding school books. There are others not on that list that I would recommend too -- "The Moth Diaries" by Rachel Klein is one of my absolute favourites, although I encountered it as a teen and I suspect it may seem a bit overblown if you read it for the first time as an adult. This year, I read "Sweet Days of Discipline" by Fleur Jaeggy, which is VERY strange, but recommended.

If you want boarding school stories for children, in the vein of Malory Towers, you can't go wrong with "The Senior Prefect" by Dorita Fairlie Bruce, or my absolute favourite, "The Plucky Schoolgirl" by Dorothea Moore.