She has had many changes in her life this year: going to a new school, starting her period, taking more interest in her appearance, and getting a lot more phone calls than ever before. I want my daughter to succeed in school. I know she is capable of the work, but I am afraid she will fail the sixth grade. What should I do?
The changes in her life -- new school, start of menses, middle grade level -- all impact her behavior and she is trying to sort out what is most important in her life. Like most kids her age, fitting in is more important than their studies, and for her it is even more so because she is the new kid on the block.
Don't stop asking about homework and make it clear that you are there to help, but if she chooses not to let you help, she is responsible for her grades and her success or failure. Then stand back and let her choose to succeed or fail. My guess is that if she fails a quarter/semester, she will make the choice to get back on track.
When you impose consequences for her behavior (not doing homework, etc.), take away the phone and computer for short periods of time at first. Make it clear to her that she is the one choosing not to do her work and so it is her choice to do without the phone, etc. Also throw in some long-term consequences. For example, "If you fail sixth grade, you will spend the summer in summer school or you will not participate in dance." You can also use positive consequences: "When you pass all your sixth-grade classes, you may spend two weeks with your grandmother or you can take dance lessons." Whatever consequences you impose, make them as personal to her as possible.
Don't despair. She will come through it with your patience and good boundary setting. The hardest thing you have to do is let her fail if she chooses to do so. Failing sixth grade is not the end of the world and I guarantee she will not do it again, especially knowing you are there to support her and not saying "I told you so!"