Have you seen these stupid posters at the airport? So you're telling me that if John Smith goes up to the janitor and asks, "Where's Billy's desk?" the janitor will say, "It's in room 304, fourth row, 2nd from the left"?
Yes, the janitor knows where a particular kid's desk is, in the same way that in a Hitchcock movie all of the guests at the deserted mansion know where the killer is--i.e. in the house with them. But they don't know which particular person is the killer.
(I know, I know, I'm getting too epistemological with the poster that just wants everyone to be an involved parent. I should stop blogging and go play catch with my son.)