1

This question already has an answer here:

Is it possible for Quirrell to have been in the DADA teacher for more a year? Could this be because he had Voldemort sticking out the back of his head?

marked as duplicate by Jason Baker, DJClayworth, Adamant harry-potter Oct 2 '16 at 3:23

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

1

As per this question, Prof. Quirrel was previously the Muggle Studies teacher. This easily enough explains Percy's recognition of him. Whether or not he took Muggles Studies, a top-level student (and top level suck-up) like him would of course know of all the staff at Hogwarts.

Not the answer you're looking for? Browse other questions tagged or ask your own question.