For me:
The Chalet School series. The early ones, when Jo was still a pupil! I brought them second hand. Eustacia, Elisaveta and so on. I remember when Jo and co. were given their prefect rooms in the new building and being in love with the descriptions/idea of it all.
Nancy Drew. Can't remember which ones, it's just a lot of investigations and stories blended it together in my memory.
The Chronicles of Narnia. I found the last book a little strange, but I read the first three over and over again.
And anything by Roald Dahl. There's a lot more (anything to do with boarding schools and midnight feasts had me obsessed) but I'm curious about what books other people remember the most.