Trebizon was a contemporary series when I read them. They are about a girls' boarding school. Much more realistic (although I know that Mallory Towers & St Clares are quite comical in a simplistic sort of way, and also reflect a bygone age) and I had a friend at a boarding school at the time, whose letters tended to confirm the books' general accuracy.

Of course, when it comes to school stories, The Chalet School gets my vote.