I really feel for her! She is paying the price for her previous, slack school. So you do need to step in and work with her on study skills -- which is not quite the same as "helping with homework."
One key thing was mentioned by TF below: Does she have a planner? In MS here, the planner is your LIFE. It's issued by the school and kids are actually penalized if they fail to have it with them in every class. It acts as their personal calendar and they are expected to write in every assignment, test, quiz, etc. If she has one issued by the school: Establish a routine, starting today, of going over it with her every single day including Fridays after school. She must get into the habit of entering everything, and including things that seem far off to her (a test next week or even weeks away, project due dates next month, etc.). Sit down and look not just at the next day's homework but at the day after that, upcoming tests etc. She needs to schedule her studying for quizzes and tests as well as her actual written assignments.
If her school does not require use of a school-issued planner, buy her a large calendar book, spiral bound, with date squares large enough for her to write in assignments. Require her to carry it daily to every single class and bring it home daily. For now, have a consequence if she does not do so. (If the school issued it, she would have a consequence if she didn't have it.) Sounds tough, but MS is tough.
She also could use some lessons from you in how to study - not the same as how to do a homework sheet or write an assignment. You may need to walk her through things like how to review notes; how to make flash cards; how to use past quizzes as study guides, etc. You may need to make some study quizzes for her to take at home -- there is nothing wrong with your creating some extra study problems in math, using her textbook and past homework, for instance. That is studying--doing it over and practicing with some new examples. You won't have to do it forever but she does need to learn skills that weren't taught in her elementary school.
Some are going to say, let her sink or swim. Nope. Not yet. If she struggles now, she is going to get turned off and start to think, "I can't do it" and she'll associate middle school with just being hard, when actually this is a time to learn great new stuff. She has two more years of MS to go, so this is the year to work with her on organizing herself and on study skills. If that means sometimes quizzing her, going over vocabulary with her, making some extra math sheets for her, that is what it takes. It's not coddling; it's teaching study skills. Kids are NOT born knowing how to study - though some parents (not you and not me) think they are, and advocate letting them figure it out all alone. It makes me nuts to hear parents tell kids, "Just go study" and then the parents are bewildered or angry when the kids don't do well -- the kids may have no real idea what "study" means if it does not mean doing assigned homework that's already laid out for them.
Do her teachers have after-school hours? In our school, teachers pick one day each week when they stay after and are available for anyone to get any help they need -- maybe the English teacher stays on Tuesdays, the social studies teacher on Thursdays, etc. She should get those schedules and use them, and not be afraid to go to a teacher every week if needed.
She will still be thinking on her own. You won't stymie that by helping her learn to study and helping her review things when it's appropriate. She needs that jump-start since the previous school wasn't up to scratch.
Does she have a schedule where she has every class, every day? Or does she have a "block schedule" with longer class periods but she only has, say, English on Monday, Wednesday, Friday, and math on Tuesday, Thursday, Friday? If it's the block schedule, she should have more time to do homework since things assigned on a Monday in English can't be due until Wednesday, for instance. But if it's an all classes, every day, schedule-- that's tough. (We have block scheduling and she and I both love it since it takes off some pressure while also creating more useful class periods where kids are there for longer.) This is why the planner could be crucial.