It was known that harry and Marietta were the only students there. Marietta was in the hospital wing, so Harry was asked for details. He did tell his story to several people. It says in the book that every single person seemed to know that Dumbledore had escaped. I wonder whether the talking portraits had something to do with it.
A piece of backstory I've been writing has snowballed into a one-shot, and I need some information about the aftermarth of Dumbledore's disappearence. I think I remember that the whole schools seemed to know things that had happened in the office, but what was it? Does it ever say how they found out?