According to a BBC Radio 4 interview, the child actors were actually caned on the hand by school headmaster (who was the real school headmaster). They were paid an additional 10 shillings or 50p (about £8.28 in 2020) for their troubles.
The scene in the library where Billy is trying to borrow a book nearly had a different ending. As they were filming with hidden cameras, members of the public could walk into the library. One older lady heard Billy arguing with the librarian and told him to 'show some respect.' Billy naturally told her to bugger off and a further argument ensued, all improvised by David Bradley and which ended with him being hit with her umbrella. The cameraman kept filming but unfortunately the film had run out part way through so they weren't able to use it. This is from a conversation with Bradley in 2017.
The headmaster of the school in the film was actually the headmaster of a nearby school.
In an interview for the book "Loach on Loach," director Ken Loach said that he regretted the portrayal of Jud in the film. Loach said that he had made Jud into too much of a brute and had not focused enough on the hardships that he faced as a coal miner.
Set in and around Barnsley, Kes (1969) was one of the first of several collaborations between Ken Loach and Barry Hines that used authentic Yorkshire dialect.
The Yorkshire dialect used in the film is difficult for some other English-speakers to understand. Some scenes were re-dubbed for the American market on its release.
The Yorkshire dialect used in the film is difficult for some other English-speakers to understand. Some scenes were re-dubbed for the American market on its release.