I love to tell middle and high school students to keep their class essays and papers in preparation for college application essays. The purpose of saving those essays is not to recycle them in their entirety but rather to keep the priceless ideas within them. The most difficult part of writing a college essay is coming up with ideas about your past and details about experiences. Those are often contained in papers written in school.
The ideas or details come from saved papers. However, a paper written for class, even a high school class on college application essay development, is rarely the essay to attach to a college application. The reason is that those essays are often written correctly but don't dig deep enough into the identity of the student. They fill in the blank on the college application but fail to convince the admissions reader that this is the student they should admit.