Wow, thanks everyone for all the replies, this has turned into an interesting discussion.
I won't have any more info until Monday at the earliest but will update when I do.
I will just summarise as there were a few queries coming up over again.
I did not confront anyone, I was at reception, while my son was collecting his stuff from the classroom at the end of the day. I asked if either the head of year or deputy head were around for a quick chat. One of them had gone home, the other was in a meeting and the TA was walking past at this point and she approached me and asked if she could help. I asked if she knew anything about what had happened today and she told me, I said thanks, that's good to know, she then said my son hadn't been involved in any bad behaviour. My son came back and we said goodbye.
All I know about what the bad behaviour was, from my son and TA, is that some boys have been mean to girls and the example was around not letting girls join in with football. My son doesn't play football. He spends his time in another part of the playground and often plays with a mix of girls and boys.
All I know about the 'punishment' is that the boys and girls were split up, and told this was because the boys had been mean to the girls, and they did different activities all day. And the boys had a lesson on respect. My son also says that at the end of the day the girls had sweets and were finishing off ice creams in the classroom and the boys hadn't been given anything.
I do talk to my son about feminism and women's rights and explain to him that still even in this country women are often treated unfairly and in some countries girls are not even allowed to go to school. I do try to raise him to have respect for everyone and he does appear to be inclusive and I've not ever seen him do or say anything sexist or racist.
He doesn't have a phone, but a lot of the other children in his year do.
The teachers who were involved in this were all male, there's actually a lot of male teachers at my son's school and I think that's great, there's great female teachers there too, and I have generally thought this school is fantastic and my son has loved it there.
I will update when I have any more information...