I think a subtle but very real element of a teacher’s behaviour management craft is the capacity to maintain boundaries by noticing issues and communicating botheredness: a firm/kind/adult tone of voice; body language, facial expressions that say, with conviction, ‘no’, ‘that’s unacceptable’. It’s calm, deliberate, assertive. Can be warm or a bit stern or even cross if needed. But you need it, whatever the backup system is. Students should know that you’re going to be bothered about boundaries. Your personal disapproval should matter to them / it nearly always does! When kids say ‘you don’t mess with Ms Smith’ it’s because she’ll notice, she’s bothered and makes that absolutely clear - in the nicest possible way.
I think this needs more explicit discussion and modelling in PD so it’s not seen as an ephemeral magic beans thing. I have met many ECTs who have this sorted already - but others need a ton of support. Sometimes it’s the noticing; sometimes it’s communicating the botheredness. It should be normal to discuss these things.