I went to that school
I went to Tonbridge School, as did David Sherwin who wrote the screenplay, which was based on his book Crusaders, which was inspired by his time at Tonbridge in the '50s. So it predated actual school shootings by several decades. More info here: http://www.blakefriedmann.co.uk/filmClients/_289/
I'm afraid the film was quite accurate: the arcane rituals, the Latin terminology, the bullying, the beatings and so on. (Actually the bullying was much, much worse when I was there in the '70s.) And we really did have a school armoury. The only surreal bit is when the boys break into it and start using the weapons. Oh, I suppose the chap in the drawer is a bit far-fetched. But somehow it seems plausible.
The producers asked permission to shoot the film at Tonbridge, but were refused.
E.M. Forster also went there and was badly bullied. He never spoke about the place and refused to go back to give lectures after he became a successful novelist.
I hear the place is much more pleasant these days :)