It's not about free speech, but having a captive audience to preach to. Dann (the father) has obviously found success in bullying and insulting people, "getting in their face" as the kids say, in the past and so that's what he wants to teach his son: grow up to be a street brawler that people avoid because he's so unpleasant to be around. "My way or the highway" disguised as "free speech".
Schools are for children to learn how to think rationally and to argue in a respectful, factual way. I am sorry for Daxx that he and his father don't "get it". Perhaps with the presidential debates on Friday, Daxx's teacher can use this as a teaching moment to have a classroom debate following the usual rules.
When tempted to fight fire with fire, remember the professionals use water.