LongAgoPlebe, I am pasting this on my highschooler's mirror. Great analysis of what grades are supposed to do. Thank you!
From this college instructor's experience, an F is not a fluke. It's a systematic failure in some habit -- studying, reading, doing homework to learn (not just to finish), not taking responsibility for one's learning -- or some systematic issue with learning -- not just learning disabilities, but challenges integrating and processing information. (Again, I emphasize I'm thinking about responsibility, not blame.) If it's the latter, an intensive course and/or a tutor (a good one) can help. If it's one or more habits, she has to be brutally honest and recognize that, then make a plan to correct it.