In Prisoner of Azkaban, when Harry, Hermione, and Ron get on the Hogwarts Express, the only nearly empty compartment they can find is the one in which Remus Lupin is sleeping. They do not recognise him:
“Who d’you reckon he is?” Ron hissed, as they sat down and slid the door shut, taking the seats furthest away from the window.
“Professor R.J. Lupin,” whispered Hermione at once.
“How d’you know that?”
“It’s on his case,” replied Hermione, pointing at the luggage rack over the man’s head, where there was a small, battered case held together with a large quantity of neatly knotted string. The name ‘Professor R.J. Lupin’ was stamped across one corner in peeling letters.
At the time this happens, the school year has not started yet, and Lupin is technically not yet their teacher (although he has of course already accepted the position at the school). Nonetheless, the name stamp on his bag includes the title ‘professor’, and is obviously not a new addition, either: it is old enough that the letters are starting to peel off.
That would imply that Lupin had earned the title of professor somewhere before the summer of 1993 when he started teaching Defence against the Dark Arts in Prisoner of Azkaban.
But where? Is there any information on whether he had at any point in the past been a teacher at Hogwarts? Or at some other school?
His Wikia article says (citing Pottermore) that after the first war, he was forced to take a series of jobs that were far below his abilities and leaving them before his coworkers found out he was a werewolf; but it says nothing about teaching.
Has JKR mentioned this at any point?