Well if you think that a boss is like a primary school kid and this is a good equivalent situation, then you live on a fairly different planet to me.
I don't recall anyone I've ever met telling a story about their boss punching staff.
Equally a couple of primary age kids in yr2 is a very different situation to a couple of yr9s who sadly may be involved with knives.
Personally, I think you teach kids to read the situation and handle it in an age appropriate fashion.
So yes under certain circumstances if you've been hit, yes to lamping back. But equally teach them at an appropriate age, that they've reached the point that this isn't going to either be effective anymore, will backfire and yes might put them at risk of knife crime.
But Yr3, when one kids decides it's fair game to lamp everyone and school do fuck all about it? Hell yes, I'm going to say just hit him back and we'll sort out the mess if you get caught, but preferably don't get caught.
Why? Because it's actually effective and there's certain kids for whom other forms of communication fail to register.