[He sighs a bit, mostly because the whole situation is entirely depressing. Though, it feels odd to be the one explaining something to a professor. Since he's been in Wonderland a long time, he's used to giving these speeches, but it's strange to know more about something than McGonagall does. He can't decide whether it's a good sort of strange or not.]

From what we've gathered in this place, whenever someone goes back home they go back to exactly whenever they were without any memory of it. So...if I haven't said anything and I wasn't missing, that would mean that at some point between now and whenever you're from, I'll make it back without any knowledge of this place. Even I won't realize I've been gone.

[None of this is good news, aside from making it home. He wants to remember it all and change things.]

Of course, the obvious problem there is that no one has any idea we're missing. Not the greatest news, I know.