OMG i remember going through this! it was a disaster for me because my mom just took all my notes from all 4 years of high school and just dumped them in a giant cardboard box and then i had to sort through it all. honestly, it's not worth it to keep most notes. you might find stuff like foreign language flash cards or notes on how to solve certain types of math problems useful, but other things you can toss out. i also kept a bunch of my old papers and homework assignments on my computer in a file just in case.
in my case, i only ended up using the foreign language stuff a little bit, but everyone is different.