Corrected entry: In chapter 23, "Horcruxes", at the end of Slughorn's memory Slughorn told Riddle not to mention to anybody that they have talked about horcruxes because Dumbledore is very strict on that point. But at the time the headmaster wasn't Dumbledore, when Riddle was a student the headmaster was Dippet.
Knever
31st Jul 2011
Correction: Slughorn mentioned nothing of Dumbledore being headmaster. He was talking about Dumbledore because he was one of the more prominent teachers that have great influence on the goings-on of the school. Much Like McGonagall is in the present staff. She's much more important than, say, Flitwick or Sprout.
Knever