Ok I have an update.
Got dragged into the classroom this morning before the bell, along with both ds's and a classroom assistant (really uncomfortable about talking about ds in front of him but she just grabbed me, so I didn't have a chance to think - she's like that)
Anyway she said she only keeps them in if they have been consistently told to stop chatting, and still haven't finished their work when they are capable of it - like tracing letters, apparently this was the time he was kept in. I know he can do that, so I guess it's fair enough to keep him in for 'two minutes' (she said) if he was just chatting. However am not totally convinced it's as low key as she made out. Poor ds was standing there as she told me all about it and I felt so sorry for him. I couldn't get a word in.
Anyway the apologising bit...she said it's not school policy, it's just her, as she likes to foster good manners and also she said, it prevents all the other children coming up to her and going 'so and so is here, mrs thing'. She said it wasn't a big deal and I told her (after being interrupted several times as I tried to open my mouth) that he found it very upsetting, and I wasn't sure I agreed with her on it as it wasn't his fault he was late, and it was humiliating for him.
There was an uncomfortable pause. This is when it all gets a bit weird. I said 'I can understand you need to know where he is, and that he's arrived, but making him apologise to the whole class seems unfair - I'd be happy to come in and apologise to them as it's my fault'. she said 'Oh!" No, it's not to the whole class!!' and then proceeded to go on at length with much horror and some amusement (?genuine) about how she would never do that, and it would be impossible to manage, and so on
and that we were at cross purposes, so of course we all laughed and decided it was totally the wrong end of the stick that had been got, and all was well.
However thinking back it doesn't make any sense does it. If he only has to apologise to her, how does that work re letting all the children know he has arrived? I don't understand that. Maybe she thought I meant he had to go round apologising to each person individually? Obviously that would be hard to manage...
so I am totally baffled now. I think actually that was the intention. I am baffled and have once more placed my trust in Mrs Thing.