anything that you think you would need that you would use as a reference to get into college, or something you want to refer back to instead of learning it again. unless your like me and only keep what your proud of such as projects and drawings and stories.
whenever I write an essay for school I make sure to keep those and my achievements/awards that I may recieve from my school.
Certificates of achievement, maybe your birth certificate, and the ending report card of each year of high school
anything that you feel you put a lot of yourself in or anything you got a perfect grade on