I think we have to redefine what we mean by "have it all." What are the standards we use to judge whether someone else is happy? What are the things we use to judge whether we feel complete and whole and fulfilled? Are we looking at superficial things, because someone had good grades and a nice wardrobe and worthwhile projects?
How do we know what "the signs" are? How do we know whether she was abused at home, pressured beyond belief to succeed, sexually assaulted by a relative (or classmate or Scout leader...), or whether she had an addiction (alcohol, street drugs, opiates from the parents' medicine cabinet or school friends), or mental illness (anxiety, depression, PTSD, bipolar disorder)? We don't.
You are in shock. You are not equipped to tell the kids anything. You have too many questions yourself. Let skilled crisis counselors handle much of this. Get a book NOW from the public library on how to talk to kids about troubling issues - ask the librarian for help, and you may have to go beyond your own library which will have many requests. See a counselor yourself to help you cope.
My husband's brother committed suicide. My stepdaughter tried twice, and has been in and out of facilities and also in domestic violence shelters/programs. She's permanently disabled by PTSD and depression. My dear friend's 14 year old son is in a CBAT facility right now - his 3rd admission - for suicidal ideation and concrete plans to take his life. It's everywhere. The challenge is to admit when we don't know much about it and don't know what to say. It's not even well talked about. We never question when someone gets cancer or appendicitis, but when they have mental health issues, we act like they should talk themselves out of it because they "seemingly have it all." It's not just you by any means. That's how our society treats mental health issues. Which is exactly why it's so hard for people to come forward when they need help.
Please urge your school to put on programs for parents too, on how to deal with this loss and how to recognize signs in our own children, and what to do about it.