So I organize stuff for a living (I'm a project manager), but back when I was a student I was terribly disorganized, scattered, and spacey! Here are some tips that may help you and your DD get through this.
Second, my DD also has a very large school and a locker in an obscure location. Not having time to drop by between each class may be accurate. But it could be that your DD just doesn't have the skills to put together a plan. For example: Go to morning classes with the red binder, visit locker before lunch, and then take the blue binder to afternoon classes. That said, my DD insisted for years that everything must be in one location. If your DD can remember to visit the locker just for her instrument, that will lighten the load considerably. I like the idea of alarms. If phones aren't allowed, maybe a watch or tiny non-connected digital organizer?
The idea of coming up with a "locker schedule" might work. She could write the new schedule in her new binder that we are going to buy as soon as the holiday begins!!!!! Thanks to all who mentioned the "Mighty Binder". I will have to suggest she write "visit band room" to get her instrument at the end of the day because she cannot put her instrument in her locker (so she says - will check up on that). The phones are not allowed in class so I am going to check up on a tiny organizer. I will check that out in a minute.
Thank you all so much!!!!!