Hogwarts is a Small School
The funniest thing about wizarding Britain is that everyone went to school with each other. And Hogwarts is small, so it’s not even like ‘I saw you once in the hallways’ its like you are at least a friend of a friend of a friend, and if we’re too old/young to have attended at the same time, we definitely know some people who did. Like literally everyone knows each other, and they should use this more often.
Prosecutor: Did you or did you not kill your wife?
Defendant: Of course not! I love her!
Prosecutor: You also loved that Ravenclaw girl back in fifth year but we all know how that ended.
Jury: *does in fact know How That Ended
And it baffles me that Harry doesn’t even know the names of some of his classmates until fifth year. My boy, there are only forty kids in your year and you have at least three classes with each of them. Why the fuck are you asking for Susan Bones’s name when she’s been sitting in front of you in Charms for the last four and a half years???
I also want to acknowledge why the actual hell they always full-name someone when they’re name-dropping them in conversation. What comes to mind is when Lily refers to Mary Macdonald, who the fandom has decided is actually one of her closest friends, AS ‘Mary Macdonald’ when talking to Snape.
Now, Mary is a common name, so the most likely reason is that there’s someone else in their year who is also names Mary. But many people from wizarding families have much rarer names, like Cormac McLaggen, and I’m pretty sure there’s not that many other people named fucking Cormac walking around.
My guess is that it has something to do with how much people refer to their classmates just by their last name. What if you called someone ‘Seamus’ when the person you were talking to had only ever heard Seamus Finnigan be referred to as ‘Finnigan’ or ‘Mr. Finnigan’ or ‘That Gryffindor in Potter’s Year Who Likes To Blow Stuff Up’.
Not to mention siblings. You’re not calling all of the Weasley’s ‘Weasley’ are you? And just ‘George’ isn’t exactly an exotic name, either.