I only have 3 kids so this may not work for a larger family, but I will share anyway in case any of it i spires you.
Through the current school year each child keeps their schoolwork in a 3 ring binder organized by subject. Most of our narrations or copywork goes in a spiral notebook which they use untill full and then I label with their name and years and save them. So the notebook is mainly math, maps, cursive.
Each semester I have to turn in report cards and I take that opportunity to clean our their notebooks, tossing nearly everything. I keep one or two examples of each thing just for fun.
At the end of the year I take any full notebooks, the dozen or so loose examples and any other noteable projects and rubber band them together. Each child has a school box (one of those plastic storage bins) I put the years stack in with previous years and it goes in the attic.
I am very careful about projects, crafts etc. I try to only have them create things they will want to use or hang up, not for the storage buckets. We take pictures of their projects for the buckets though.
So my hope is that by graduation they will have a bucket with several full notebooks with their writings, drawings, nature notebooks, a few loose samples of math, and some photos of fun things they did. I do generally save workbooks just because they cost so darn much, but I try to avoid them as a rule so there arent many of those taking up space. They would be he first to get tossed if I start running out of room in my buckets.
I dont know how helpful that is, but that is what is working for us so far. My oldest is in 7th grade and we still have room in his bin so I think I am on track keeping to one bucket per kid.
Good luck, it is certainly hard to toss work. I feel like it sends a bad message.