Please help me save my best friend?

You need to realize that while doing what is best for her you might also alienate her. If you can't trust the adults at school try to think of an adult that you do trust. At church, or even a police officer, child protective services someone that can get her the help she needs. She has a history of a suicide attempt maybe that will make them take your concerns seriously. Once you send up the flare be prepared for the fallout that may or may not come. Remember that you are seeking help for a beloved friend and her wellbeing is the most important. Be there for her and continue being the supportive friend she needs.