The best aspect of this film is the spot-on parody of English public schools, which could only have been created by people who have been to them. There are so many brilliantly portrayed character types in this film, from the grungy eco-warriors to the public school kids to the complacent headmaster. It's hard to pick out one particularly good actor as they were all convincing and hilarious.
I think the negative reviews may largely be from Americans, because they would have missed so many of the social references and parodies. For a film with plenty of silliness, lots of the humour is very English and very subtle.
I absolutely loved it.