If all facts are as they are being presented, this is clearly a very bad and very unjust decision. Students, like all Americans, ought to be entitled to an appeal before fair-minded, reasonable authorities. Zero-tolerance usually implies zero-intelligence as well.
On the other hand, I just heard the family representatives on TV bemoaning that the whole incident will destroy the girl's chances for college admissions and a collegiate volleyball career.
Those assertions are absurdly over-stated.
I read university admissions essays every winter and I guarantee that if she crafts a savvy essay about this situation emphasizing her moral obligations to keep a friend safe at the sacrifice of her own ambitions it will be a positive element in the admissions process.
As for volleyball, if she is really good she will get a spot on a team. In the high school volleyball world the major club tournaments are the important showcase for college recruitment, not the school seasons. And a single glitch in the legal area like this will not be an issue... take a look at the criminal records of any BCS football program.