But Serbian children, who seem to be less bothered about fairness, believe he is a good teacher - because he is strict.
The cultural difference in children's views about what makes a good teacher was uncovered by Amy Shuffelton, of Wisconsin-Madison university, who taught 10 to 12-year-olds in Serbia last year.
Serbian youngsters do not care that Snape, the potions master, unfairly deducts house points from Harry and his friends in Gryffindor house at the drop of a pointed hat. Instead, "strict but not too strict" was how most of them liked their teachers, she told the American Educational Research Association's annual conference in Chicago.
Professor Dumbledore, Hogwarts' headmaster, was rated the best teacher by all the Serbian children, because he is sympathetic, supportive but "not too indulgent".
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