And once we’re on the subject of Percy:
If it is true that there is a jinx on the job since Voldemort applied for it, it must have been Quirrell’s first year teaching. How would Percy know who he was?
I first thought that prefects were given introductions to the new teachers beforehand (in the prefects carriage on the train?), but it is evident in the Order of the Phoenix that prefects are not informed of staffing changes.
It could be that they generally do inform prefects in the carriage on the train, but there was an exception for Umbridge. Remember, the Ministry only have a right to appoint a teacher if the acting Headmaster cannot find an option. Dumbledore was probably waiting until last minute to confirm the position. (Although Defensive Magical Theory was already on the booklist)
It could also be that Quirrell was an exception to the rule. It sounds like Quirrell had been teaching for more than a year already when you look at Hagrid’s introduction: