I came across "Old Boys" by accident on British TV when channel-flicking, and luckily just as the Continuity voice announced it was a film about a public school (so I had none of the pre-conceptions that other IMDB reviewers had).
For a time I wondered about in which period it was set, thinking that it might be the early 20th century, so it was a surprise when a 1980s Volvo appeared. In fact much of the film sought to recreate the sort of public school and its bizarre rituals that might have existed a century or more ago, and certainly nothing like the one I attended in the early 1960s. We would not have reacted to "The Dam Busters" in the jingoistic way that the boys in the film did. There was a palpable sense of lust when at the end of term my school screened "Some Like it Hot" with Marilyn Monroe in a very tight dress,)
Pauline Etienne was sweet enough as Agnes, but Agnes Laurent as and in "The French Mistress" (the 1960 film about a young woman accidentally appointed as the French teacher at an English public school) was more likely to have aroused emotions among pupils and staff).
A scene near the end reminded me of Tom Courtenay's act of defiance in "The Loneliness of the Long Distance Runner" released in 1960.
The cast and settings were excellent - Lancing College, where filming took place, must be one of the most imposing of Brittain's public schools, with Eton, Marlborough and Winchester all having less conspicuous buildings that do not catch the public eye.
