Despite the difficulty involved in setting fair grading standards, a red flag goes up indicating problems when an honor student, like your daughter, suddenly starts getting F's in just one class. It may be that her English teacher has no idea of what the typical standards for work at this level are in American high schools. There is also the possibility that he is having problems explaining exactly what he expects from the students.
You have expressed your concerns to this teacher. The next logical step is to set up a meeting with the chair of the English department. It would actually be best if your daughter and a group of her classmates discuss the teacher's grading with this person. They must not just complain about the grades they are receiving. Instead, they need to bring many samples of their failing work to the meeting. The chair should be able to see at once if this teacher is employing unreasonable standards. If so, the chair should be able to work with your daughter's English teacher to help him grade more fairly -- an important consideration because grades are so important in the college admission process.