Wow... what a hard start.
I'm really in disagreement with people who suggest that the school punishment was 'enough'. A sincere, face to face apology is necessary, if only for *you* to build trust with that teacher so that she knows you aren't going to brush it off. Many people sweep their child's actions aside, claim that kids will be kids. Yes, they will be children, however, if we set reasonable expectations for them and give them good guidance, they will do better because it is expected. So-- if it were me, apology, no tv/media games for the rest of the week. This may not be convenient, however, he needs to learn that when he threatens someone in authority, it is not a joke, not something that we brush off-- that he is in BIG trouble. This is serious.
As a preschool teacher, I would have had the child picked up immediately or suspended from school the following day for making such a threat to me. Could a child hurt me? In reality, I have been hurt by children, usually on accident. So while the reality is that it isn't likely (perhaps) that your son could have hurt that teacher, is it *more important* for the child to understand that threatening someone with bodily harm is completely unacceptable, as you wrote? Absolutely. (And read my previous posts-- I am NOT a 'bring the hammer down' sort of parent. But this is so serious...)
It's time for a family meeting and laying down the law about what is and is not appropriate. Your SO/husband NEEDS to be made aware of what's happening, and all the kids must follow your house rules. Make a list of approved viewing and if that list is strayed from, then ZERO television privileges. (TV is a privilege, not a right, as far as I'm concerned.)
That said,your kindergartener also needs some reinforcement from you that NO MATTER WHAT he hears, if he knows it is wrong, hurtful or mean, he is responsible NOT to say those things. You are putting a bit of this on your stepchildren, however, the fact of the matter was that your son *did* use this as a threat, which leads me to believe that he did know what he was saying and directly challenging the recess teacher's authority instead of cooperating. That he made the choice to do this cannot be pinned on anyone else but him. Your whole family needs to have a rule that threatening is never, ever okay.