English ability is very important. She needs to be fluent for the interview to be accepted to some of the best schools. Many of the top boarding schools will not take students for G11 unless the student's English ability is close to a native English speaker.
One thing that you might want to consider, is that your daughter is young. Due to the one child policy, children in China are generally less mature when compared to American students of the same age. You may want to repeat a year to make sure that she learns social skills.
Many of the top schools will require PSAT or SAT when the student is applying to G11.
