Organization is the absolute keystone to having a strong academic career. Here are a few quick things you can do with your child -- or if he/she is old enough, to tackle alone!
Grab your backpack. Dump allllll the paperwork out. All of it! All the binders, everything.
Go through it all. Make piles for classes (Social Studies, Math, etc.), a "keep for me", and a "throw out" pile.
Every paper, even ones IN the binder, are pulled out and examined. "Do I need this? For what?" If you do, put it in the pile. If not, get rid of it. If you're not sure, get it out of your bag and keep it in a folder on your desk.
If your kid doesn't have finals, don't keep old work. Let it go! If he or she does, you may want to put it in a safe place for future study sessions.
You should have a much tidier bag now!
You can repeat this with any workspace -- desk, cubby, etc.