Most people, friend or classmate, know that I play osu!. Mostly because I played it excessively last year in school, but it's on and off so far this year. About four other people that I personally know play osu!, and most of my friends know about my, ahem, "osu! addiction", which nobody can really call it that at this point, since I don't focus on osu! 24/7 anymore.
So yes, they know what osu! is, since I play it.