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NankerPhelge Calling people by their surname is frowned on and generally not accepted by society.
With few exceptions (e.g. Galileo, Napoleon, Rembrandt) we tend to refer to famous people by plain surname but I agree we don't usually address each other in that manner. I believe US teachers address their students as Mr/Miss <surname>. I don't see the necessity of that level of formality but at least it's equally respectful to both sexes. Prisoners are also referred to and addressed by plain surname, perhaps that's why teachers address boys like that.
As I'm not a christian, I don't have a christian name, I have a "first" or "given" name. Some societies still use mononyms, I believe Iceland is one such.
I don't see the logic in that at all.
There's is little logic to many school practices. They make rules just because they can, not for any practical purpose. They often seem to be in competition to come up with the most stupid rule. One, fairly local, primary school banned children from wearing facemasks because they're not legally obligated to. Now the teachers want to be given priority for COVID vaccinations because they're at greater risks - due to their own stupid policy! Another school banned a student's "designer" face mask - It appeared to come from Aldi's Winter Collection as it had the word "Aldi" in small print at the bottom one side.