I wish I knew how to help you. We tried to prepare dd for my moms death, but it doesn't really work
You never are prepared, even when you know it's coming and I think it's even harder for kids.
I would just be honest with her and tell her that he's sick again, and that the outcome doesn't look good. Answer any questions she may have as honestly and truthfully as you can.
Good luck and I'm sorry about the teacher, how sad for him, his family and his school/community.