Interesting topic! The truth is I am happy I gave my charrie a canon last name because the link between her and the HP storyline makes me more eager to RP! However, Griselda Marchbanks was only mentioned in the books, so I could still make up an original backstory which doesn't conflict with anything Jo wrote. And I have yet to meet any other Marchbanks in the school RPG.

On the other hand, I would never use Weasley, Potter or Malfoy as a last name, simply because those families have been described in detail in the books and any new character should comply with the existing family history. Plus, as underlined by people before me, it would be very difficult to keep track of all the relatives your character supposedly has in the school RPG. No need to complicate ourselves, I'd prefer an original character with an unherd-of name to a Weasley any day.