Hogwarts - dangerous but magically wonderful - doors that weren't doors at all, solid walls just pretending.
But before that one was around, what about the school that the naughtiest girl went to - I forget what it was called but the students had a hand in running it. Wasn't it just awful when Elizabeth finally finds something that she likes about school - music but it gets taken away from her as a punishment.I suppose it worked though.
I read all the Malory Towers and St. Claire's but not all of the Chalet School as our library had a rather sparse collection dotted throughout the very long series.
Still has to be Hogwarts for me, but I did like those other ones when I was younger.