It is shocking. I'm not surprised you want to talk about it.
Something like this happened with my ds, though he was considerably younger than your dd and I had known before he heard.
I talked to him about it and it became clear that children gather what they are capable of from information like this, and not much more. So, in my ds's case, I left it at a fairly general chat to establish what he knew, what he thought and some very general stuff about "feeling uncomfortable" and bodily integrity. As I said, he was a great deal younger. I did keep checking in with him, though, to see how the other child was being treated in the class. It seemed that the class dynamics just ambled along pretty much unchanged.
I suppose you are going to have to have a more in depth chat with your dd. I, personally, would be interested in how the other girls' friends are handling it - both the impact of the break-up and the possible abuse.
It is awful. I was thinking of the other child just the other day, and thinking just what you have said - that some children have to carry terrible burdens very young.
I think TiggyR and BennyandSwoon are tight - in a way, it is only in time that the real impact of what your dd has been told will become clear to her.
And I'd suggest that you might well be in some degree of shock now. I certainly noticed that, with myself, I thought about the incident at odd times and each time thought about it slightly differently, probably as the shock i felt wore off and the impact sort of made itself clearer.