In Boy, Roald Dahl draws some interesting comparisons between sweetshop owner Mrs. Pratchett and headmaster Mr. Coombes. For one thing, neither of them seem to like children very much. For another, they both enjoy inflicting pain (or at least watching someone else inflict pain). For a third, neither of them are interested in anything remotely resembling fairness or forgiveness.
Dahl and his friends definitely do wrong when they put the dead mouse in the candy jar in Mrs. Pratchett's store. She hurries right over to the school to identify the culprits and ensure that Mr. Coombes punishes them properly. It is clear that Mrs. Pratchett does not like children. She is always scolding them and thinking that they are going to steal from her. Mr. Coombes, despite his job, doesn't seem to care much for kids either. He immediately assumes the boys' guilt and punishes them without even asking for their side of the story.
Mr. Coombes's punishment of choice is caning, and he does it hard and long. Each of the boys gets several sharp whacks that really hurt. No pleading will soften the blows, and if the boys stand up, they receive more blows. Mr. Coombes seems to like what he is doing, and Mrs. Pratchett certainly enjoys watching. She cheers the headmaster on, yelling, “Lay it into 'im!” and “Teach 'im a lesson!” This spectacle seems to have made her day, maybe even her month.
Finally, Mr. Coombes and Mrs. Pratchett don't care about fairness or forgiveness or helping the boys see why what they did was wrong. They care about punishment, and that's it. The boys have never stolen from Mrs. Pratchett and don't intend to, so her judgment of them is wrong to begin with. And as for Mr. Coombes, even when Dahl's mother goes to speak with him, he doesn't relent. He goes “ratty” with her and tells her that she can take her son away if she doesn't like the methods at his school.