Butterbeer does contain alcohol, but not a lot. We find out as much when we see Winky at Hogwarts in GoF. Ron, I think, remarks that Butterbeer contains barely any alcohol, and Dobby replies that it's a lot for Houseelves.

Concerning teachers and alcohol at mealtimes – I agree that it wouldn't be common during the regular lunchtimes. However, I somehow remember a Christmas party with Mc Gonagall getting slightly tipsy. Or is that just my odd memory? I think she is giggling and blushing, and her hat is slightly askew, or something like that.

Edit: woot, spider post. This should be no.5, after obsessed-with-jo.

Edit2: Ha! got it! This is Christmas in PS, "The Mirror of Erised":

Harry watched Hagrid getting redder and redder in the face as he called
for more wine, finally kissing Professor McGonagall on the cheek,
who, to Harry’s amazement, giggled and blushed, her top hat lop-
sided.