Let her choose an assignment book/planner on her on. It may be giant, it may be lined and cute, it may have tabs, but let her choose what she wants to keep her school at a glance together.
Also, how about a second set of BOOKS at home? You may have to buy them secondhand on-line, but the savings on her back! would be worth it! Ask the school if they have extras. I know my very organized kids have sometimes forgotten a book. If there is a 2nd set at home, no worries!
Does she like mechanical pencils? They always made me feel more neat/organized.
And, a general checklist for her locker each day--
Math daily homework? upcoming test? any projects/reports? Supplies needed?
English daily homework? upcoming test? any projects/reports? Supplies needed?
Science daily homework? upcoming test? any projects/reports? Supplies needed?
History daily homework? upcoming test? any projects/reports? Supplies needed?
and so on.
Just seeing the words and reminders for each class can jolt action.
And, allowing her to fail now, before HS and permanent records, could be a good thing. Do NOT rush assignments off to her school. the headmaster at our private school said, "letting a child fail when young can teach them much." Now, stay in touch with the teachers, also. If DD says she is turning everything in and the first quarter report is dangerously low, have a conference with the teachers and find out what DD is missing. Not copying boardwork? Losing folders? Talking in class? Missing papers? Daily grades or quizzes or homework issues?
If she has done her work, every now and then get her an unexpected treat. Be encouraging, but not helicoptering.