Spoiler Alert: Please don't read if you've not already seen the film.
This film is a classic in horror mystery genre. I was not expecting it to be so brilliant. It really felt timeless. It might seem slightly predictable because of the passage of time since its creation--many times the formula has been repeated and now it has become a cliche--still, it's certainly a masterpiece.
Such brilliant horror mysteries are really rare. There's no gore, no real blood-shed and a tense haunting mystery after the interval until very end of the film--and even after you've finished watching it--you keep thinking about the details. I think it would not be wrong to put this film in only mystery genre--but from the viewpoint of headmistress it was indeed horror as she suffers from a weak heart condition.
The film is very well paced for its nearly two hour long run. Performances are pretty solid and it has an air of comedy too. The mystery is indeed tight and since unlike many naive-everything-on-your-plate movies the details of execution of crime-mystery are not revealed which makes it really intriguing for the audience.
I had predicted the twist quite well--but since the death of the headmaster was shown very clearly--it kept me guessing--further the dead body was put in the pool and stayed there so it was also another blow against the prediction of the twist. Anyways, it was way better than usual horror movies which leave little to imagination and have almost no suspense about them. This film is perfect by most standards in my opinion. I loved watching it. As I've discussed in previous posts--this movie also belongs to those great twisty movies which change your entire perception about the story in the last five minutes. Since this movie has been ripped-off too often you might find it too predictable and nothing matching to your expectations but it's similar to The Wicker Man, Skeleton Key and other such films--in this film you find that though it seems that it has all been about killing the cruel headmaster of the small French school, but actually it was all about trapping the headmistress who had a heart condition. In these movies victims often seem aggressive action-oriented individuals and then suddenly they're trapped as preys.