There's no reason to suppose he wasn't at another magic school previously. Theoretically, people are teachers because they are trained/have experience in teaching, right? I'm not sure how that works in the wizarding world, though, what with the centaurs and the Trelawney, etc, but...you know. It's possible he could have taught for a while before everyone found out he was a werewolf, or in a place poor enough that they didn't care. After all, only Dumbledore and Snape knew when he was at Hogwarts...nobody knew when he taught at Hogwarts.


Failing that, I'm going with the being lousy at name-stamping charms.