HLBOES Ghost Quiz

Well, we've been set an assignment to identify how many ghosts are resident at Hogwarts. This is a tough assignment, I mean I can name several, but what counts as ghosts? Do all the past heads in the pictures in Dumbledore's office count along with the other picture residents throughout the rest of the school? And what about Tom Riddle, who appeared sort of as a ghost when Ginny got caught up in his diary? There is definitely reference to 20 ghosts streaming into the Great Hall in the Philosopher's Stone, but I think that is probably just scratching the surface. So my answer to this assignment is going to be "it depends upon your premise"! But to show willing, here are the names of the 7 resident ghosts I know of for certain: Nearly Headless Nick; Peeves; The Bloody Baron; The Grey Lady; The Fat Friar; Moaning Myrtle; and Professor Bins. And now Witching Hour is here, must dash...!!!