DTs are year 2, will be 8 in September. There's been a couple of incidents with DTS2 pulling another boy's shorts down to show his underpants (because "it's funny" 
and "everyone does it") - we talked through it, explained it was inappropriate, if he does it more he'll be in big trouble etc. Thought he'd got it.
Had a phone call from school yesterday to say that DTS2 and the same boy had been rubbing their penises together (through clothes) and obviously this is a big flag for them that something may be wrong. The safeguarding lead has spoken to both boys and said she'll have to think what's best to do next, how they could help etc. I am absolutely mortified 
I've spoken to DTS2 who said it was the other boy who suggested it - but of course the other boy says it was DTS2 so we have no way of knowing! I explained again that even if someone suggests it he doesn't have to do it, it's not right regardless, etc etc... DTS2 says he understands but I don't know, I think he's just so desperate to be friends with this boy that he'd agree anyway. The other boy is a couple of months younger but much more popular, confident etc and DTS2 is sad whenever they have a falling out.
I've thought hard about whether DTS2 could be copying anyone - or be subject to this himself - but he's rarely on his own with another adult male, not even DP as he works long hours. He goes to sports clubs but again, isn't left on his own, it's always in a group with other children and adult supervision.
Anyone have any ideas how I could help more - or what school may do?! I'm on tenterhooks waiting for bad news, I just have no idea what they might put in place. DTS2 isn't behind at school, on track with all school subjects etc, it's only this which they have any concerns about and I don't know how best to help!