Headcanon

werewolfmcwerewolf:

Remus always told the other Marauders to do their homework and
was always the one to warn them about getting into trouble whenever they wanted
to do a prank. So the other Marauders start to call him professor Lupin as a
joke and it sticks. On Remus’ birthday one year they all buy him a small case
with Professor R. J. Lupin stamped on it. Remus took this case to Hogwarts
every year until he left. This is why in The Prisoner of Azkaban Remus has a
small battered case with professor R. J. Lupin stamped on it in peeling letters when he has never
been a real professor before.