Hi, I agree with momtofour, however...
I think part of why you are having a hard time bringing it up with her is that it really should have been a discussion between the two of you first, even if you are 110% sure of what happened.
It just seems like a trust thing, and an area where you might have missed an opportunity for her to come forward herself if you both determined that she did plagiarize.
I think the best thing is to just explain what you found, asap, what you think happened, and that you already called the school and then let her talk.
I am hoping this won't sound too negative.
