Preschool teacher here, the way your sons teacher is going about it sounds wrong but for me I have to be wary of my students teaching each other potentially rude phrases. Like, one kids parents might be totally cool with their kids saying "what the heck," "oh my God," "shut up," or using a lot of potty humor at home, and other parents might be upset by that. (Not to mention saying poop at the lunch table is a sure fire way for a table of 3 year olds to be so overcome with laughter that they can barely be redirected back to their lunch. Actually someone saying poop anytime outside of the bathroom is a good way to lose all control and management of the class lol)

So it's on me to explain that saying "what the heck," or "oh my God," or talking about poop while eating might be okay at home, but isn't polite at school.