kids and swearing

The other day I got a phone call from my son's school saying that he had used the "f" word when another student slammed his binder down on a lunch box. My son is in Kindergarten, so this is not something that is common. There had been a substitute in the classroom all week, so things were not the usual routines and procedures. The principal told me that she felt that my son didn't really know what he was saying, but when she told him that it was a bad word, he got embarrassed and almost started crying.
When my husband first heard about it, his first response was anger, but then he realized that punishment probably wasn't the best choice in this case. Instead, he surmised that how can a child really know what words they aren't supposed to say, if no one ever tells them. So when my son got home from school that afternoon, my husband sat him down and told him all the words he's not supposed to say. Yes, I do mean all the bad words that are considered bad. Then my husband told him that if he says those words, he will be in trouble.
So we'll see how that goes, if my son, who now knows all the bad words that he's not supposed to say, chooses not to say them, or if knowing that he's not supposed to say them, chooses to say them anyway.
:/