Students from grade four onward tend to store their papers in binders. As adults, we assume that the use of a binder comes naturally and is just common knowledge. For some people that is true, but for others, it does not come naturally, and is a skill that needs to be taught.
The easiest system for most students who travel between classrooms is to have one large binder that holds everything. They always have everything with them, and there is less chance of forgetting what they need for a particular class.
If only one binder is being used, it needs to be a good one. The kind that you buy at the grocery store for a couple of dollars will not hold much, and will not last. Get a sturdy, large one that will hold up through the year. I find that the fabric-covered zipper types work well. They often have the added bonus of compartments for pens etc. inside the front cover, and when zipped up, prevent things from getting lost.
Some even have file pockets for carrying homework papers or notices.
Colourful dividers keep the subjects straight. Make sure your student knows how to insert papers into the correct area. Sometimes, they just open the binder randomly and jam a paper in, without looking to see if it is in the right subject area, or (if it is) in the correct order. Yes, really. Someone needs to show them that it works like a book; the new chapter goes at the end.
A “pocket” for each subject is also helpful. There can be homework papers, projects in process, flash cards or file cards that won’t go in the holes.
A page protector or plastic pocket will hold those things safely.
If the binder does not have pencil/pen loops in the front, a zippered pencil case with just a few crucial tools (pencil, pen, eraser, small ruler) will help the student get to class with all the important stuff.