I first saw this film many years ago on late night. So it was the chopped up cut version. However, even back then it left an impression on me. I remember that the mystery of who the killer was, was so good, that I didn't even know until close to the end of the film. The movie has alot of adult themes and was so ahead of its time, it's amazing. The basic plot is that the school is really a sort of reform school set in the late 19th century.
So the girls of that time are considered bad girls. The heroine ends up there, I believe, for running away from home. Soon after she arrives, girls start to disappear and she finds a certain amount of lesbianism going on at the school, too. And she meets the son of the headmistress. I don't want to give away too much of the plot, but this movie is really a must see for Argento fans. Try for the uncut version. You'll remember this movie for a long time to come.