We've been in school 40-something days, and I've finally managed to train most of my 26 students to reply to my daily morning greeting. I always tell each one, "Good morning" when I see them at the door. One would think they would know what to say in return, but they don't until someone like myself teaches them.
I've said these words many a time:
"So tomorrow when I see you at the door, your goal will be to say 'Good morning' when I say it to you. Let's all practice."
I love it when some of them arrive and beat me to the words. Of course, I make a big deal about that, because it's a contagious kind of thing that spurs others to follow suit.
At this point, if someone doesn't say it back to me, I say, "That's when you say..." and they'll remember the appropriate reply.
I just think it's important for little people to practice social awareness and respect. They don't know what that looks like until someone teaches them, and oftentimes that someone is their teacher.