Don't blame yourself for what happened it's not your fault, I mean how could you possibly know she was dying like you said you where on the bus while she was outside. It's a sad that the girl passed away but it's not your fault at all, things happen and there's nothing we can do to stop it. It's very common for people who have experienced death before there eyes to feel guilty and it's even more common for someone to feel like it was there fault, If you still feel like it was your fault then I suggest talking to an adult that you trust and if that doesn't help talk to the parents of the girl, say that you're very sorry for there lose.