I'd be interested to have a teachers perspective on this one.
DS is Y3. He came home rather disturbed saying that he'd been sitting on a lunch table with Y5/6 boys he didn't know, and they'd been talking about having sex 'and raping people'. DS says he ignored them and they ignored him and said it was all 'disgusting and very inappropriate' or words to that effect. He has ASD- type difficulties and has been in trouble a few times himself for inappropriate stuff at the wrong moment, but this was evidently way out of his league. I asked him if he knew what raping people meant and he said no and didn't require further clarification (to my relief). He has an age-appropriate understanding of sex - ie he has a very broad brush understanding of the mechanics, knows it is something adults get up to in order to have babies,and has no idea why they should wish to indulge in such a weird and disgusting activity
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I'd prefer DS wasn't party to more mature conversation than I think he's ready for, but not the end of the world. And I would expect 10 and 11 year olds conversation to be pretty earthy, so a chat about sex wouldn't surprise me hugely. But if your Y5/6s were having lunchtime discussions about rape would you want to a) see why the subject had come up and b) explore a bit more what rape actually means and why it isn't an appropriate subject for dinner time chat? Am wondering whether I should mention it to the school on that basis.
This is all assuming that DS hasn't got totally the wrong end of the stick and is a reliable witness ...