I get this question every now and then. I always say that I don't. Mostly that's true. Children are in my classroom and then they aren't. I was an adult when they knew me and adults don't change all that much. But children go through puberty and change a lot.
But sometimes when they try to explain who they are, it's worse than that. I've had a couple of tall teenage boys tell me I taught a sister or brother. Well, that means that I saw them occasionally when they were in preschool or kindergarten or something...not memorable.
Today I got one of those questions. I don't remember ever seeing the kid, but I remember the brother when he was in the fourth grade...which means if it had been the brother, I wouldn't have recognized him, either.