I don't know if I'd be quite that strident (and I recognize you wouldn't really be, either). My conversation with my kid would depend a lot on how the teacher was, otherwise, but assuming this is the first/only incident, I'd probably say something like:
"Look, Mr. Dickless is a good teacher, and I don't want you to change how you approach him or his class. But, realize that even people we look up to are sometimes wrong. This is one such case. Just know that you will be faced with situations in life where someone you respect, or someone with authority, is wrong, and how you handle it is an important skill to learn as you get older. You must always try to be the bigger person, and sometimes that means keeping your mouth shut, and sometimes it means taking a stand. What do you think you should do in this case?"
If the kid says "This is really important to me, teacher was wrong.", I'd say he could talk to the teacher after class and express his disappointment. If he says "it's just football, really, who cares" then you praise him for keeping correct perspective when his teacher clearly didn't.
Either way, it's a learning experience. I don't think I'd contact the teacher more than once (if at all) as a parent, though.