What is a correspondence school?

A correspondence school is a school that provides education to students through distance learning. Instead of attending classes in person, students receive their coursework (lessons, assignments, etc.) through mail (traditionally) or electronically (now more commonly) and submit their completed work to be graded, usually by mail or online. Essentially, it's a form of distance education predating the internet and online learning platforms we know today, although the term is still sometimes used to refer to schools that primarily utilize mailed materials.