I lost a classmate in 7th grade to anorexia, she just never came back from Christmas break. It was so confusing for all of us, we knew she was sick (she looked horrible), but many of the adults didn't want us to talk about it. (thankfully, times have changed)
I think one thing that is critical is that the kids be told the truth, they need to know what happened and why. I am not saying scare them with gruesome details, but they are going to hear tons of conflicting rumors, they need someone to make some sense of it all...if at all possible.
Another thing that is a common occurance with MS/HS kids (dealing with a classmates death) is that there will be kids who use this situation to get attention, they make up false stories and sometimes spread hurtful rumors. IMO, it is largely due to gross immaturity, but it can be very hurtful to those who were really close to the person who died or the family.