No, not at all upset. It’s important the girl’s classmates know, so they can support her. And so she doesn’t have to explain it over and over, which can be retraumatising.
I’d then talk to them about it too. Death shouldn’t be a mystery. We all have different views of death, so the school can’t explain that. They can only give the facts. So, it’s important we talk it through as parents.
I’ve always spoken to my kids about death, in an age appropriate way, as it’s an unavoidable part of life. People close to them have died. Great grandparents, grandparents, close family friends, some of their close friends’ parents (several mums as well as dads) and siblings, pets. They’ve always come to funerals from a young age. IMHO it helps prepare them. I wouldn’t want the first funeral they attend to be someone really close (even me), when the whole experience is a mystery and therefore much more stressful.
As I’m not religious, I’ve drawn parallels with death in nature, e.g. how plants die and then new plants grow. I’ve reassured them that I won’t be dying any time soon, so they don’t worry. When my eldest had to have lifesaving surgery, the youngest, who was then 8, asked if he was going to die. I had to be mindful that he may and didn’t want to give false assurances. So I said probably not, as the doctors were great and he was strong. He might do, but I felt he’d be OK. Pleased to say he was.