This also happened in my child's year 6 class recently. The boy had no repercussion whatsoever, he was talked to by the headteacher, that's all. The boy who really looks 13 grabbed a girl under her skirt and she wasn't wearing tights. She was so upset.
To make things worse, I mentioned this to another mum with a son in the same class. She said, boys will be boys, they're just children messing around, the girls was being dramatic.
She is such an apologist and saw NOTHING wrong with it. Ironically she also has a daughter in year 4 and we all know that her big brother beats her up to the point she has bruises. This is a naice, lefty middle class mum in a naice leafy school with lovely job. Did I mention that this mum is a proud feminist? Sick.