When I was still in high school in the Netherlands, there was a teacher, still very young, 24 or so, who sagged pretty low. Sometimes students commented on it, then he would just say he is used to doing it from when he was younger. Then he would just go back to teaching without pulling his pants up. And even though some students commented on it, they didn't really care. It's pretty relaxed here fortunately!
But I'm assuming you live in the US, so I don't know how they will respond to sagging teachers there.
About smoking, I feel like there must be a teacher on every school anywhere