Good manners

Ben and went back to Gan (kindergarten) last week.
He's not the type who says much about his day. Only after probing a bit, or even better, telling him a little about my "exciting day" at work (what I had for lunch is usually the highlight), he'll say something like "Our teacher brought a real shofar to Gan the other day" or "I didn't like my tahini-and-cucumber sandwitch, so I didn't eat a snack today" (And so, guilty of treason, I cross out another "nutritious option" filling and make a note to buy more "round yellow cheese" with the hope I'll manage to add another item into the menu before the end of the schoolyear)
But today, something seemed to bother him.
"Ma, did you know that people in my gan don't close their mouth when they eat!"
Really?
"Yep. Not even the teachers!"

I chuckle. What am I supposed to say? My loud, hyperactive (sure seems like it) four- year old, the one that hardly says hello and goodbye, and who knows that whining works much better than please and thank you - worried about this?
Should I answer "You're right" and implicitly criticize his teacher ? Should I tell him that all people are different and have different style for eating? Or say "c'mon, it's Israel, son, and everyone is doing their own thing"

So I don't say anything. I just smile. He was listening to what we say at the dinner table after all.