Coming from an Ontario high school with a graduating class of, I dunno, 300 or so, 350? I probably at least knew the vast majority (90%+) of my class in terms of names and faces. There were tons of kids I never had a class with, but again, you usually at least know who the person is, even if you've never spoken to them before. If I had my yearbook around, it might be fun to dig it out and see how right that guess is.
But as this topic shows, I guess it's totally different for other folks. And if my grade had 600 people instead of 300 I'm sure there would've been lots more kids I didn't know.
That said, when it comes to SOTF I usually prefer to have my characters at least know who the other person is in a thread, even if it's in terms of "Larry didn't know Wilhelmina all that well", because as I've said before, introducing-each-other scenes are godawful and horrible and boring wastes of time to write and read.