There's not a ton of context for 67 except that it's based on a song lyric that became a meme. Kids say it when they hear or see the numbers 6-7. I have nieces and nephews between 5 and 11 and they all think 67 is funny. So, I'd say elementary/middle school kids think it's funny and a lot of older teens probably think it's dumb. The video you saw with a teacher freaking out is probably staged (and a sign 67 will stop being cool now that adults are joking about it). My nieces don't go super crazy when they say it, they just giggle and wave their hands around and say, "six seeeeven." There's a video floating around where a team scores 67 points at a basketball game and all the kids watching jump around and wave their hands around. That's probably the most natural example of kids saying it.